Really good question..The most would be $650 for a Sexton, and a few other less known for $300-$400. Now, any extra income is spent on work by photographers here on APUG.
Why, well the first criteria that has to met is - appeal, why buy it if you don't really like it. Sounds simple enough, but not sure people always do that. Example, I would not buy an Adams print just because it was an Adams print. Since, IMO the golden age of photography, as we know it, is over and traditional prints (silver and alt process) will become less common, the work of those here that offer it for sale is about as good a purchase as any collectible.
Why not higher prices, well that is hard to say. Some of the members here are selling plt/pld work for what I consider way below market prices. At the big auction houses, there are lots that do not sell, because the minimum is not reached - at the same time there are works that sale for much more than the estimated prices...go figure..such is the world of collectors.
In the end, it seems there are at least 3 types of collectors - the first, with the deepest pockets, are what I call the true collectors. They collect as an investment, some of the works they collect may never see the light of day. The 2nd group would be the general public, they may not be collectors in the truest sense, but they want something to hang on the walls of the office or home, to let others know they have good taste, etc. The 3rd group would probably be those here...they know quality work when they see it, the produce quality work as well sometimes, they have limited resources to expend on artwork other than their own. If given a choice between paper/chemistry/etc or buying a print from a fellow photographer they will probably buy the supplies they need. Some may even think - "I can do that" and some will even go as far as to say "I could do it better", but they never do and never buy a print either, sad to say.
So, the best thing we can do is support each other...will we get rich from the work of a member, probably not, but we can have the satisfaction of owning a well crafted print, that is way above the average prints people buy and display in their homes/offices. After all, if we were doing this for money, we would probably be doing something else.
Just my 2 cents...your mileage may vary.