You are right in that Gainax never passed on the 
cels to wholesalers like many of the other 
studios. Instead, many of the Gainax staff were 
given the cels and some were destroyed. The cels 
price are therefore a function of rarity and the 
secondary market (ie collectors like you and me) 
having to wait for a Gainax staff member or 
friend (lucky recipient) etc to resell some of 
the stuff they have.

Some dealers like Asylum have much better means 
to tracking the right sources down.