Book prices are all over the place. I purchase what I want, and typically at "fair enuf" prices. Some of my books have supposedly increased in value, some have supposedly decreased. I don't really care. I didn't purchase them for an "investment".
I get a LOT of mileage from a good book. Decades worth. Neat thing about books, is they can be read/enjoyed, then go to the shelf for ?? years, only to be retrieved at a much later date and enjoyed all over again. Plus, they are fantastic for "throne time reading". 
Also they are invaluable for research/etc. I have books covering all different types of regions, era's, railroads, etc. Love 'em!
Bottom line: It's up to each individual to decide whether a particular desired book is worth what one gives.
Andre
