Hi there,
I believe they are worth quite a bit, you may try 
mailing this place, it's a animation gallery in 
Houston, they deal quite a bit in these cels.  
Also, another place, called Van Eaton Galleries 
deals in a lot of SON cels.  They would be able 
to give you a good estimate I think :)

Fine Toon GAlleries @
http://www.finetoon.com/cgi-bin/finetoon/

Van Eaton @
http://vegalleries.com/warner.html