For the last 18 months I have been standing at coin fairs selling antiquarian, second hand and out of print numismatic books and what has struck me is the reluctance of people to actually buy many of these works.
I can understand that going to a fair you might be more drawn to a new coin purchase, history in the hand as it were, and for the price on some of the books the coin might, at first, appear better value but without the books you are buying blind.
What governs the price of numismatic references? I suppose new ones the amount of work that has gone into the publication, the research and knowledge it contains, the fact that many works aren’t big sellers so set up and print runs are more expensive per unit cost and so on.
Why should a second hand numismatic book cost so much? Again it must be judged on the knowledge/information it contains, is it available elsewhere or is the work the reference for the series, how many people want it and how many are available to meet that supply, given that above we have already acknowledged that some of the references are in extremely short print runs and there may not be an alternative reference for the series. That is always then assuming that it is not an antiquarian book.
The long and the short of it is that many of the most detailed and up to date resources are expensive, but also specialist, and many of the "normal", general, collectors are not willing to pay out significant amounts of money for them.
It depends ultimately how detailed, I suppose, that you want to document, record and understand the coins in your own collection as to how much you will pay for specific references.