Well, one of the problems is that there aren't 
that many
anime cel dealers on the Internet that have the 
Very Expensive
Super Rare kind of cels because they're 
expensive to keep in stock (like diamonds, I 
reckon ~_^)  At conventions, one often sees 
very expensive/rare
cels that aren't for sale on cel dealer's 
websites (Usagi's House and 
Asylum Anime had some of these)  At Otakon, 
I believe that UHouse had
a Tenshi ni Narumon hanken for $850, as well 
as some sketches by
various Japanese artists (also for about $850)  
At one point in time,
Animanga cel shop had a Firetripper hanken.

Gaudiora