From what I gather, they look at the current 
market (What they cost on ebay, in japan, other 
stores), and make an approximation based on how 
popular the charecter is, how good the shot is, 
if it has a background, etc.  I'm sure the reason 
many use "evilbay" because if they set up a 
store, it wouldn't get the same attention, plus 
you might get more then you expected... hopefully 
this information is useful, and correct 
somewhat :).