I was pretty impressed with AA this year, both the location and the general quality and variety of material available. And frankly, the prices I saw in there for prints were pretty comparable to other conventions I've been to; nice big prints, especially the ones on good glossy paper cost the artists a decent chunk of change to print, whether they have their own printing equipment and buy the paper to make their prints in-house, or use some printing service to produce them. I quite happily got 3 nice large prints for $40 and considered it an extremely fair price for what I got. I've also come home with a collection of artists' business cards for possible future commissions or purchases, since of course at-con I had a somewhat limited budget.
I couldn't speak for commission prices since I didn't personally get any, but as others have pointed out, you are paying for a custom-created piece of artwork. You are not simply buying a piece of paper that a printer spit out a digital file on, you are buying that artist's time, skill, potentially years or even decades of practice and education that goes into being able to make something one-of-a-kind just for you. That stuff shouldn't come cheap, because artists need to make a living and shouldn't have to undersell what their skills are worth just to appease people who want cheap art.