Ok, I'm a BT'X fan but I don't have any cels 
from this series. If you consider that most of 
the shops sell common cels of this series 
without sketch and bcg for $15, I'd say this 
gorgeous cel easily worth $50, maybe $75.

Hem, by the way, if you intend to sell it to 
another person for any reason, then, think of me 
^_~ But I do think something between $50 and $75 
would be a VERY reasonable and realistic price 
for your friend. So go ahead!

Good luck anyway!
Almathea