This is a very good question to bring up.  
Ultimately, it is your decision on whether or not 
you wish to give up the cel.  If it's something 
you're going to regret later (no matter the 
price) don't do it.

I wanted to bring up a point, many of the 
responses mention that there are a lot of 
collectors who are in the money and are willing 
to toss out obscene amounts because they have 
nothing else to do with it and that the amount 
doesn't mean they're desperate for the cel.  I 
can agree with that to a point there are several 
collectors out there like that; however not ALL 
of them.  Many of my dream cels came from offers 
from another gallery.  A couple of those cels I 
did offer an extreme amount for (waaay more than 
what the cel is worth) and I am hardly rolling in 
the money (I just graduated with a BFA in art, 
moved, and was recently married = no $$).  
However I was willing to pay that price because 
the cel was that important to me.  Ultimately, 
the price is up to you, whether you want to sell 
it or not, but I wouldn't immediately assume that 
the offerer is shelling out the cash just because 
he or she can.  Usually you can determine the 
case (whether the offerer is truly after the cel 
and not the transaction) through their email.  
Hope this helps!