In some ways, I quite agree with you -- I firmly 
believe that you should take your own finances 
into account whenever you make a non-essential 
purchase, cels or otherwise.  And that's not 
just the big-ticket items, either -- if spending 
$40 on a cel will make you nervous about 
affording your gas bill or buying that required 
textbook, it's really not worth the stress.

I may make a few jokes about living on ramen and 
selling plasma, but that's all they are -- 
jokes.  Both hubby and I are at the age where 
our careers and finances are relatively stable 
(or as stable as anything gets in the videogames 
industry ;), our mortgage is low and our car is 
paid off, we're not having children, and we can 
therefore afford to treat ourselves from time to 
time.  A bit more often if I'm willing to resell 
art that I'm no longer quite as attached to, or 
collectables and electronics that I no longer 
need.

When I first started collecting cels, about a 
decade and a half ago, I was quite happy to buy 
one inexpensive cel every couple of months, 
because that's what I could afford without 
impacting the rest of my life.  These days, I 
indulge in quite a few more, but the overall 
percentage of spending to income is actually 
about the same.  The difference is that an 
editing and writing contractor with ten years' 
experience makes just a wee bit more money than 
a temporary file clerk who's fresh out of 
college.  *grin*

With that out of the way, though, I have to say 
that I view cels and production sketches as 
works of art, and as such, their value is 
determined solely by their attractiveness and 
importance to the buyer.  One of my seven 
biggest wishlist cels cost about $45, another 
was nearly $700, but I value them equally in my 
heart and would have paid more for either if I'd 
needed to.

I once heard an illustrator say that, "all art 
can make you think, but good art makes you 
dream."

And dreams are worth a lot to me.  :)

Peace.