well,

this is all just my opinion, but i think that the 
different series / movies / games definately are 
a factor.  for instance, i would definately be 
willing to dish out big money for primo OP or OAV 
shots of Ryoko, but not really willing to go for 
TV series shots (especially Shin! Tenchi Muyo) of 
her because while the shots may be the same 
quality (close up of face, torse shot, etc.), the 
artists were different and she doesn't really 
look the same.

i also think that movie shots of ryoko especially 
are more expensive because she's kind of drawn 
different each time, especially when you look at 
her in the third movie.  hope that helps,

haruna