The price of a books varied quite a lot depending on the quality, size and binding. A family bible intended to last generations could be quite expensive. I've uploaded an example bible cover here, which shows it at 6 shillings unbound for a reasonably nice edition. Converting prices is problematic, but here is a list of comparisons, which also mentions Samuel Johnson's dictionary at 10 shillings.

Phillip Gaskell's A new introduction to bibliography is a good starting point for reading up on this.

I remember listening to the Thomas Jefferson Hour, and host Clay Jenkinson said a book cost around $140 in the days of Jefferson (a little after 1750).