I tend to set limits on myself by series (eg with 
Bebop its $500.00 per cel with Tenchi-Muyo its 
$200 - With Utena its virtually anything).
I set these limits on how much I like the series 
and characters.

On many occasions my internal spending limits are 
very different to other collectors who love the 
series more (FY is a prime example - which is why 
I have not bought anything from that series for 
yonks). I either have to lift my pain treshold to 
meet the market or simply look elsewhere until 
the madness dies down.

In other series I have been prepared to spend far 
in excess of other collectors. God knows how much 
I'm prepared to pay for cels of Juri from Utena 
(Keys are you listenning?).

Different series have very different price 
structures. No-one seriously shells out heaps of 
loot on Naruman cels despite the fact its a 
fantastic series (Do not get me started on DBZ). 
FY cels are amazingly expensive for 52 Episodes 
of TV plus OVAs.

Remember how much people used to shell out for  
cels from certain series? Prices for these have 
come back quite a bit from the heady days.

Patience is a virtue.