I was just wondering what the costs of a book would be before the invention of the printing press. For example, how much would a hand written book cost in relative terms. Would they be considered priceless objects hundreds of years ago owned only by the wealthiest nobles, or would a well off merchant be able to buy one? Also, would the content of said book influence the price at all, for example a Bible versus an ancient play? Thank you for your time and effort in advance.
This is going to depend on the size of the 'book', the writing material, its accessibility in the area, whether you wanted to get it illuminated (made all fancy looking), and time period. Even then this is still a rough ballpark, but the short answer is it's a bit like a modern-day car.
Books made of vellum started with the quire, which is basically a the lambskin eventually getting folded into a ~sixteen-page pamphlet.
I don't have my main price guide handy, but this page gives a few examples, including one from early 14th century Paris, a bit under 2 pounds for a 27 quire / ~430 page book. The 8 pence illumination is pretty basic - I distinctly recall some absolutely ridiculous illumination costs (on the order of hundreds of pounds) for some books.
According to Standards of Living in the Later Middle Ages by Dyer, a thatcher would be earning about 3 pence per day in this period - so this is the better part of a year's income for such a commoner for such a book.
Books getting sold later on didn't always fetch this price - though I don't have much information about size or content in those sales so it is hard to make a judgment.
Note that not all steps need to be paid for at once. A comparatively well-to-do family could save up for the parchment for a Bible, then get the copying done, then get it rubricated, then get it illuminated, over the course of several years.