Another issue: How often are new books released? And as a longtime consumer of gaming, how often are new editions released?
Ex: Chaosium puts out a new hardcover for Call of Cthulhu about 3-5 times a year which run $40-60. I can deal with that and don't buy every release. The current edition of the game is also around 10 years old. No talk I am aware of for a new edition. An edition of a game lasting 10+ years feels good and stable for me. I used to buy a LOT of White Wolf World of Darkness games, and some of their lines got new editions in under 3 years. Crazy.