Well, the most I have ever spent on one cel is 
$550.  I never regretted it for one moment.  
It's one of my best cels, and one of my most 
treasured.
But as far as your question is concerned... you 
have to ask yourself if the cel would be worth 
losing all that money.  I like to think of it as 
a huge negative, and if it could be countered by 
a big enough positive of the cel.
I myself can think of a few cels that (if I had 
the money) I would shell out $XXXX amount of 
monies.  I would feel crazy and deranged for a 
few months probably :P but it would be worth it.