It depends on the series and character a lot. But 
also keep in mind that certain cels are more 
meaningful to a scene with the eyes closed. 

Also, some character almost always have thier 
eyes closed...i.e. Xelloss... chichiri... 
So cels with their eyes open jump significantly 
in price.

For the most part - eyes open cels get more money 
compared to eyes closed. 

And for finding out how much to ask for cels? 
that's the tough part. You could maybe post the 
images here? 

My favorite eyes closed cel: