I would think there are AT LEAST 5-10 times more 
collectors for DBZ cels than X-movie collectors.
A simple search of Rubberslug Galleries would 
show that to you.

Cels prices vary due to local preferences for 
characters. A cel of a particular character may 
be worth more in Japan than the US - examples I 
can think of are:
Sasami from Tenchi Muyo
Tomoyo Daidouji from CCS 

I also would be very careful of accusing someone 
of lying.