Let's start from the observation that although the prevalence of gold coins in fantasy is pretty ubiquitous, with the 'gold piece' being a basic monetary unit that sometimes is divided into 'silver pieces' and then 'copper pieces', most often than not using a decimal system that is very modern invention (just to drive the point home, it has been only adopted in United Kingdom as late as 1976) this hardly reflects the realities of the medieval European currency. The purchase power of the medieval population was also very different from the one observed today due to differences in self-reliance across social classes and present economic mechanisms.

First and foremost, it should be noted that the gold as a currency has generally not been used throughout most of the Middle Ages in Europe. It doesn't mean that it hasn't been used for trade, as it had the estimated value to evaluate the objects made of this metal, but it was simply not used to make coins. This only changed in early 13th century with the augustalis introduced in 1232 on the order of the Emperor Frederic II being one of the first if not the first officially used golden coin, clearly inspired by Byzantine hyperpyron or Sicilian tari. Even then, they were not widely used and have been initially minted only in Brindisi and Messina. Golden currency slowly started to gain popularity only few decades later, with the introduction of the florin issued in Florence in 1252 and zecchino introduced in Venice in 1284. Popularity of gold coins surged in 14th century after the discovery of rich gold deposits in Hungary (Kremnica mine) that later proved to be the largest source of this metal in continental Europe, estimated at five times larger than those of Bohemia and Silesian duchies (contemporary leaders of continental gold production) combined.

The golden coins minted since 14th century Europe more or less emulated the florin, leading to a rough uniformity among the golden coins used in circulation. The original florin and zecchino weighed about 3.5 g (55 gr) and were of high purity, usually reaching 995 (99.5% pure gold) and this was reflected in other medieval golden coins that followed suit (including the 7 g double florin issued by Edward III in 1344). There were, of course, differences, such as the Dutch Gouden Leeuwen (Golden Lions) minted since 1361 that had a weight of 4.25 g, while the late-14th century golden coins in Burgundy weighed between 4 and 4.2 grams. Thus, we can assume that a prototypical 'gold coin' would mean 3.5 g of gold.

How much purchase power would it mean? Well, this is very tricky question, as the actual gold-silver ratio, purity of coins and last but not least, the prices themselves could have varied quite significantly between decades not to mention centuries, but given that this is a highly generalized question with no territorial and chronological frame, an equally generalized response can be provided.

When first issued in late 13th century, florin was made equal to a pound of silver that was divided into 20 solids or 240 denars, following the ancient Roman system reintroduced during Carolingian Renaissance and quite prevalent across Europe, with the last vestige of it being the aforementioned pre-decimal British monetary system. Given that the Florentine pound (lira) was more or less equal to its ancient equivalent (libra) equal to 327 grams of silver [gAg], it means that it could have been divided into 240 denars containing 1.3 gAg or 20 solidi containing 15.6 gAg.

In the beginning of the 14th century a silver coin commonly used in Central Europe was a Prague grosh that at the moment of introduction by Vaclav II around the year 1300 was equal to 3.5 gAg, although later in undergone quick devaluation and in the beginning of 15th it contained only around 2 gAg. Now, if we compare the value of the coin to the common prices from the early 14th century expressed in Prague groshes, we can see that the price of an ox averaged to 30 groshes (105 gAg), a ram or pig - 8 groshes (28 gAg), a horse could commanded a price of 100-200 groshes (350-700 gAg) depending on its quality, a well-made large cart could have commanded a price of 2-3 marks (400-600 gAg) and a lan of land (ger. Lahn) equal to 40 acres of 16 ha was valued at 192 groshes (672 gAg) in one of the contemporary documents from early 13th century Silesia, with other land prices in this time and area roughly following suit.

This said, a comparison of wealth across various economic mechanisms is not as easy as one could imagine. The main problem is the fact that said mechanisms meant that the distribution and usage of currency could have been vastly different. For example, in modern capitalism, virtually all goods and services are exchanged for money and people, especially in urban settings, are completely not self-reliant. In rural areas where feudal relations was the norm, the opposite was true - people were largely self-reliant and in theory could have lived their entire life without making a single transaction involving money. Peasants had to pay for the land they were using, but the rent this could have been issued in nature (work and part of the products and this is how it initially often worked in Early Middle Ages) but usually did not have to pay for their houses, fuel, maintenance, food and simple tools as they were able to provide everything themselves. This meant that peasants could have a substantial personal wealth even if they had little currency readily available, what makes comparisons rather difficult. Using the aforementioned prices from early, it can be clearly seen that a peasant owning 4 oxen (a norm for a homestead, people owning less animals often paid smaller rent on an assumption that they are unable to till much land) had already a wealth equal to 400 gAg, i.e. more than 1 florin from the 1290s. If you then factor in a horse, few pigs, a cart (necessary to transport grain and other products and provide transport services for their lord), it can be clearly seen that a peasant family on a well-prospering homestead could have a property amounting to several florins of equivalent gold currency. So, it can be said that should the peasant liquidated their assets mobile assets (land was usually his master's property), he could have easily earned several 'gold coins' (i.e. 10-15 grams of gold).