I think it's not necessarily the rarity of the 
cel...but the fact that people like the character.. 
.  I mean Keiichi goes for about $40 while a 
similar (size, orientation..etc) bellandady goes 
for at least $200 (and that's if you are lucky).  

  I know as a collector...I care less about the 
rarity and more about the character..and what 
is going on in the scene.  Opening cels and 
key cels are often more expensive because 
supposidely the animators spend more time 
(and more prominent people work) on 
time...but then again...some opening cels are 
not very interesting.  

Oh...well...