It varied greatly, as you might expect. In a day when every part of a book was handmade, the price of a book was heavily dependant on the cost of labor. We're not just talking about the scribe who actually writes out the text; the paper or parchment, the leather cover, the binding, any illuminations or illustrations, all had to be done by hand. This meant that, as the cost of labor fluctuated, so too did the cost of a book.

But we can get some hard numbers. This article contains inventories of books held by 14th century English instructions, including their values (the article is on JSTOR, but did you know you can get 100 free articles on JSTOR right now?) These were books that could be loaned out to students, and the article notes that their listed replacement values may be somewhat inflated to discourage students from losing or stealing them. Nonetheless, it's as good a place to start as any. In the early 14th century, the books in the inventory ranged in value from 24 pence (two shillings) to 360 pence (1£ 10s), with an average value of 97 pence (8s 1d) (remember that until 1971, English/British currency was 12 pennies/pence to a shilling, and 20 shillings to a pound, for a total of 240 pennies to the pound). By the late 14th century the average value of the collection had increased to 196 pence (16s 4d), likely reflecting an increase in labor costs brought about by the plague.

Also, note that your 8 shillings in 1300 or 16 shillings in 1400 wouldn't buy you an entire bible. There are no complete bibles listed in the inventory. We're talking about things like commentaries on a single book of the bible, or a collection of various works of Augustine, or Paul's collected epistles.

Of course, those values are for common books intended to be loaned out to students. High quality books could be much more expensive. The same article refers to a set of two antiphoners (books of liturgical music) which were commissioned in London during the late 14th century for £24 5s 2d. Even if we consider each of the two volumes to be its own book, valued at £12 2s 8d, they were still priced at more than 100 times the value of the cheapest book listed in the second inventory above.

Ok, so how much was 8 shillings in 1300? Or 24 pounds in 1400? Well, comparing prices and wages across centuries with any degree of accuracy is basically impossible. Wage labor was not nearly as common in the Middle Ages as it is today, and what people spent that money on, and how much of their money they spent on which things, has changed dramatically in the last 700 years or so. The Late Middle Ages was still a time when most people were agricultural workers of some sort who often didn't earn or spend money at all on a daily basis. But for a skilled laborer in the early 14th century, like a mason or a thatcher, a commonly cited wage is two pence a day. That means that in a good year, this person could expect to earn around two pounds, or 40 shillings. So the cheapest, smallest book would still represent more than two month's income. In the years following the plague wages went up (assuming you survived it, of course), but even if your wages had doubled, so had the price of books. As for the large, illuminated books commissioned by cathedrals, universities, or the wealthiest of the wealthy, our skilled laborer might work his entire life, saving every spare farthing he made, and still not have enough to buy one.