considering that I've been collecting Card Captor 
cels since last summer, i'll put my two cents in 
and hope it helps some...

Sakura cels are more expensive than kero cels 
since she does take way more paint to make than 
just a yellow "stuff animal" but that is not a 
set rule. It just depends on the cel itself. If 
you wonder about the price of a Sakura cel, just 
ask yourself how much you want it. Cel prices are 
very subjective. Usually opening cels are VERY 
expensive, like the response before said. But 
costume cel prices vary depending on how 
nice/cute the cel is. If you're not sure, i would 
just suggest surfing around different CCS cel 
sales site and comparing prices to get a general 
idea

if you have any more questions, feel free to 
email me too ^_^