Hmm....
I think Asylum anime sold one for $400 - or was 
it more than that?  Hmm, might have been closer 
to $600...  It was of Tamahome punching Nakago 
for the last time, at the end of the 
series...with a background.
Another cel I saw and wanted very much was one of 
Miaka and Tama embracing from the last episode, 
also with background.  The price was ridiculous 
though- small full body shots with no faces 
showing, and I believe that one was close to $500.