There are many facotrs i think that contribute to what you charge for example here at renderosity you could sell a model for 50 or a 100 dollars do to the volume you sell you could easily make 8000 to 9000 dollars im guessing however if you are selling at a sight that deals with a few sales at a time youd need to charge a much higher percentage for example if neededd to make a realistic hand or arm etc for a hospital i could easily justify doing the work and charging say 1000 dollars because id need to cover the time it took to make from one sale only ! however if i sold the same model here i could justify selling it for 10 and hoping to get 100 sales the market of course is a factor as well there are few here that would spend 8000 for any model at any cost however a ad agency geting paid 50,000 for a 30 sec spot wouldnt be as concerned many other things come into play as i said these are just a few