Cost of printing Press
Before the invention of the printing press (and the switch to paper for pages), books were very expensive because of the cost of the labour involved in copying them out and the specially treated animal skins (parchment) needed to make pages.
Parchment, which needed to limed, scraped and then dried stretched out under tension on a frame could represent one third of the cost of the book, according to Jonathan Bloom's book, Paper before Print The History and Impact of Paper in the Islamic World (New Haven, CT: Yale UP, 2001). While these figures are for the cost of a book in Byzantium (now called Istanbul), European costs would probably have been similar.
Around 1100 AD, a Latin psalter with canticles that took four weeks to make cost 53 shillings to buy (Intertexts: Studies in Anglo-Saxon Culture presented to Paul E. Szarmach). At about that same time in England, you could buy a warhorse for 50 shillings (The Knight in History, Frances Gies), a laborer earned a maximum of 40 shillings a year according, a kitchen worker earned up to 6 shillings a year and even a chantry priest earned less than 100 shillings a year ( London in the Age of Chaucer, A. R. Myers).
Even an inexpensive book, like the 53 shilling psalter, was a luxury item before the invention of the printing press.
Update: The question I answered has been merged, so I'll do my best to answer the second part of the question.
With the introduction of the printing press, the cost of a book fell by 80% (H.E. Bell, The Price of Books in Medieval England). Still expensive, but within the reach of the middle class.