There are two guidelines by which I spend on cels:

1.)  Level on my wishlist/quality of cel
2.)  Monetary expenditures available.

Basically, how much do I want it, and more 
important, how much do I have to spend?  
Currently, the most expensive cel in my 
collection was $300.  On average, most of my cels 
cost me around $100.

As a very prelimniary basis, I don't let myself 
go much beyond $300.  I'm willing to make 
exceptions for certain series - chances of 
finding good VHD: Bloodlust cels cheap?  HAH! - 
and characters/scenes that I really want.  For 
example, my ultimate wishlist cels, the ones for 
which I have specific screen shots?  You could 
easily get me to drive up into late hundreds for 
those.

Now, there are other factors to take into 
consideration, too.  The average price range of 
the series, for example.  I would never go for a 
cel listed for over a $1000 whose series usually 
sells for about $200.  In example, I love Fushigi 
Yuugi to death, I really.  And I like Nuriko, but 
I won't ever buy a cel of him.  He's way too darn 
expensive for a character that I'm not all that 
passionate about and whose design I don't find as 
appealing as other characters.

Similarly, it depends on the rarity of that cel.  
Is it an opening/closing cel, a hanken, a shot 
from a hard-to-find OAV or general series?  Would 
you ever be able to find it again?  Are you 
willing to bid like hell to get it?  For example, 
I collect G1 Transformers cels.  They are RARE.  
I've only managed to pick up two in the three 
years I've been collecting, and they've cost me a 
pretty penny.  But y'know what?  If it's a 
character I like - BAM!  I'm going for him/her.  
Those are tough to find, and I know I might not 
get a chance to find them again.

Lastly, is this a cel that will stay with you?  
Will you not regret spending so much on it?  Will 
spending so much on it ruin it for you?  My most 
expensive cel is Fushigi Yuugi's Suboshi.  It's a 
gorgeous cel, from a specfic screenshot that I 
wanted.  He was more than a $100 than I'd ever 
spent before and is the one responsible for 
knocking my limit up a bit more.  But you know 
what?  I don't mind.  I still love him to death 
and smile every time I see him.  He's such a rare 
character, I've only seen about three or four 
cels of him otherwise, and even better than a 
cel, I picked up a friend.  So it's definitely 
worth it to me.  :-)

All this is stuff you need to take into 
consideration when you buy this.  When it comes 
down it, it's really whether you want it that 
much mixed in with financial ability.  Good luck 
in deciding, though, they're tough choices to 
make.

Oh, and btw, my "push the limits" baby is 
below. :-)

-Chaotic Serenity