i think it really depends on the cel.  

If it's a bank cel, i suppose that's about 
average.  If it's fer from an image of her from a 
rare scene, then that could be a good price, 
depending on the scene.  Also, which sailor moon: 
regular, super, or eternal?  Do you know any 
other details?  

It's pretty hard to judge by just that 
description.  But in the end, if you or someone 
else would be willing to pay that amount then 
that would be the retail price.  if no one is 
interested in that cel for that amount then, it 
could be overpriced.  Try asking, other SM 
collectors.