I second Alex's reasons above.  Also, I think a 
big factor is that Cowboy Bebop is still popular 
in Japan. Japanese collectors are willing to 
shell out big bucks as well.  So for once, it's 
not just the deputy services driving up the 
prices on Y!J. ^_^;

However, there has been a recent "renaissance" of 
Cowboy Bebop cels. I've been collecting them for 
about two years now have more than doubled my 
collection in just the last few months.  Maybe 
now that they are becoming more available the 
prices will come down a little.  This past 
weekend, there was a fairly nice Ed cel that sold 
on Y!J for 20,000 yen. In September, there were 2 
that sold for over 100,000 yen. O_o

--rg, resident Bebop addict