So we cannot really talk about a "cost" of war horses as we would understand it today is a purely monetary value, and I cannot say that a horse in 10th century England would be the equivalent of X amount of dollars/pounds with any sort of confidence. We have records for the prices of goods, including horses and coconuts throughout history, but these are not always easily translatable as we operate in entirely different economic systems than prior societies and these records would also be uneven after a certain amount of years. Today the cost of an item is usually the price that we pay for something, this is usually based on the demand for the item, the supply, the labor involved in its creation, transportation, and so on. So in short, its impossible to give a clear answer on this topic. Good day!

But...

I cannot give you a dollar amount for either good, but I can describe the processes that would be involved in 10th Century Britain.

So let's start with horses.

Horses are weird. They are immensely useful beasts of burden, but simultaneously a potent status symbol for the upper echelons of society, as well as food source. In our specific context of 10th century England, horse consumption was rare (having been legislated against by the Church) but we are also in a time period before the creation of systematic breeding programs designed to produce the largest horses. Indeed we are at a time before the codification of a "knightly" elite in England at all, with the mental image of an armor clad horseman. While cavalry was not unknown to England at this time of course, textual evidence indicates that dedicated forces of cavalry were rare, and the English famously fielded no cavalry against the Normans in the 11th century. That is not to say horses did not exist within martial endeavors. Horses were still needed for transportation, carrying supplies, pulling wagons, and so on, even if their use as a force on the battlefield was not as heavy as it would be in later Medieval times. All of this really means that a war horse as we might envision a knight, or a rabble of knights and their lords, riding might not actually be all that different from a horse used in other pursuits at this time.

Evidence for horse keeping is a bit spotty at this time in English history as well. We know they existed certainly, but we lack details about how they were raised, where, or for what purposes. I imagine horses would be unusual to see on the average farm but would have been relatively common in more elite contexts, despite their lack of intimate connection with landed knights at this point. (There is more to delve into between the relationship of horses and England, but that's a different topic) However there is a great deal of effort involved with raising horses for military purposes. The animals need to be kept on pasture land, expensive, they need to be fed with produce from other land, also expensive, they need to be trained, they need to be maintained with shoes and shelter, not cheap either. All in all, even one horse was a significant investment, even if a precise dollar amount isn't easy to arrive at.

But what about coconuts? Coconuts as we all know are tropical, and England is a temperate climate. They do not grow indigenously, even in Mercia. Coconuts are indigenous to South East Asia and the areas on the Pacific coast particularly, today most are grown in Indonesia and the Philippines. In their husked form they have a shelf life of a few months, so is that enough time to get the coconut to our hapless British knights in time before it rots? Well maybe.

Long distance trade at this point in history was undergoing a revival following the collapse in the emporia trade of the early Middle Ages and the depredations of the Vikings and other raiders. The Carolingian court famously sent embassies to the Abbasid Caliphate which had trade ties to China and India going back some time. This trade was largely in luxury goods though foodstuffs almost certainly were included as well. It is conceivable that a coconut could make its way to trade centers in India from the islands in a matter of weeks, and then a month or so to the markets of Baghdad or Damascus after being purchased by Arab traders. However this leaves us with a mere month to reach England. Two weeks would be needed to reach Marseilles, Pisa, or Genoa, leaving another two weeks to make it to England. Its unlikely that any trade could get from the Mediterranean to England overland in this time, but going by ship would be possible but probably not before the nut has rotted.

However assuming that the coconut has reached London before rotting completely likely with only a few days to spare, how much might one person pay for it? Unfortunately we have no real way of knowing. We might assume the price would be high due to the long travel time, but since coconuts were not really a luxury good, beyond their inherent exoticism, its unlikely it'd be an exorbitant price, certainly not more than the costs involved with raising a horse and the agricultural produce needed for years of growth, the land to keep them, the labor in maintaining them, and the training they would require.

I can speak a little to the coconut, but less to the warhorse. Perhaps someone with a heavier equine focus could answer that and we could put two and two together.

The coconut palm is honestly, a pretty cool plant. The hardy fruit can survive several weeks at sea and is pretty well dispersed among almost every coastline near the equator, avoiding some, like North Africa, and loving others, like the Caribbean, and Pacific islands. The early history of the coconut was tied with the early people who gathered and used them, probably a people in Melanesia, Indonesia, or the Philippines, where genetic studies have indicated it is likely that coconut palms originated. Either through an epic months long ocean float, or via human activity, they ended up on the pacific coast, and eventually all of the tropical Americas.

In the 2017 historical fantasy epic Moana, in the song "Where You Are", the villagers helpfully describe the various properties and uses of the plant and its fruit. Consider the coconut. The coconut was an immensely useful industrial crop providing fuel for fires, and a robust fiber that could be woven into rope or clothing. It was also a useful agricultural product, providing water and food stock. The modern coconut palm, known for its use as a stallion substitution, has been domesticated, selecting for thinner husks, higher copra (the white meat) content, and higher water content. Indeed, if we could only figure out “Where We Were” when coconuts were first domesticated, it could provide a great deal of data on the spread of people around coastlines worldwide, as coconuts are well preserved in fossil and genetic records, but I digress.

We have a crop that was common basically everywhere there was a subtropical coastline by the 10th century, which is widely cultivated for it's incredibly helpful industrial and agricultural properties, and a huge interconnected network of trade routes stretching from England down to the rest of the world. India would have easy access to cultivated coconuts, and might have been the closest source of an enterprising anglo-saxon looking for an impromptu horse replacement. While areas of eastern Africa and the Middle East both currently produce coconuts, only a few spots in Oman had coconut palms grown in large numbers at that time.

It’s possible that a coconut made its way to England in the 10th century. Fresh, they only last a few weeks. Dried copra can last a few months. If we’re just talking about the shells, obviously they could be taken as a curiosity by a merchant and hocked out to Europeans, but there’s no specific evidence of this. That being said, from the start of the Indian-Europe trade way back in Roman times, it would have possible to get a coconut to Britannia. I personally find it hard to imagine such a useful good not just incidentally making itself way up from Egypt (where Indians definitely traded them) to England in a millennia of time.

However, if we wait a century or two, we actually do have concrete evidence of coconut palms in mainland Europe! Due to a quirk of style, and culture, several enterprising European nobles thought it fashionable to turn coconut husks into goblets, bowls, and mazers. These were gaudy, ornate things, overlaid with silver and gold. We’ve got a few from the 13th century, and many more onward. There’s also good reason to think there were less valuable cups made from coconut, inlaid with pewter or copper.

So, sometime around the 13th century, coconut shells became common enough to woodwork with into cups. I’ve been combing through the National Archives to find a reference to a coconut cup in a will or debtors note, but I’m coming up empty so far. My suspicion is that it was much cheaper to acquire the husks as opposed to the metal inlay. At the very least, knowing a price for a coconut cup would provide a high estimate for the acquisition of a suitably horsey noisemaker, if you were to buy such a goblet, throw away the gold, and keep the just the coconuts (obviously).

This is, arguably, a pretty poor way of estimating the price of a coconut husk, but unfortunately, barring a lucky record of import or purchase from the time, there’s not much else we can do but guess. It’s a phenomenally tricky task to price things out in the past without concrete primary sources, and even then, there’s layers of now lost traditions and culture that hold the actual reason for that price. You probably wouldn’t just guess that coconuts would make for vogue luxury goods. Once you get away from the utility of a good, who knows what a superstitious person would have paid for a “magical” “mystical” shell of a distant tree he could scarcely imagine. Valuations, more than most other aspects of human life, are especially vulnerable to fancy, imagination, and the whims of historical society.

I believe the quote by Publilius Syrus goes “Every coconut is worth what its purchaser will pay for it”. As best I can say, a coconut husk was worth (probably significantly) less than an ornate mug.