As Cordelia said, hankens are prized, as they are 
one-of-a-kind, used for promotional purposes. I 
would also like to add that the fact that the 
lines can be hand-drawn, and often the cel is 
drawn/created/painted by a senior animator or 
someone trusted and involved, but with 
experience. This often kicks the value up. 
Basically, hankens are the -original- artbook 
pictures, etc.

-Misha