The fuzziness, I think comes from the wierd 
overlap that occurs in hobbies involving 
collectables: Are you an investor or a 
hobbyist? Yes, you can be both, but that's 
where the dilemma and fuzziness occurs, if 
you separate them, I think it is a bit easier:

a) Investor. How do you justify? By studying the 
market and making a best guess at whether 
this cell's value will rise. If you believe it will, 
voila, you've justified it. If not, there you go, 
don't buy it.

b) Hobbyist: How much to do you love the cel 
in question? Enough to pay X for it? If so, 
that's all the justification you need.

Assuming a hobbyist/I do this because I like 
to, perspective, that's why I ask: Why do you 
need to justify it? You don't, so don't. 

People who enjoy fine wine will pay 50, 100 
dollars a bottle ... and boom! Its gone, drunk, 
but a memory of pleasure. People who are ski 
nuts will fly to Colorado or Vermont, or Europe 
even, and spend several thousand on a  week 
or weekend of fun. Gone, enjoyed, but a 
memory of pleasure.

No object will bring you true inner peace... 
blah blah blah, BUT, if you really love that cel, 
and you're not spending your kid's lunch 
money or the rent, well then, who cares? Your 
satisfaction and happiness is all the 
justification you need. Don't spoil it with regret. 
If, after the fact, you feel you spent too much, 
then that's valuable, too: Next time you will 
have a better idea of what your limit should be 
for each level of like (i.e., oh, nice, wow, 
amazing!, *drool**drool* pleeeease! me 
want!).

It's when we cross this simple, common 
sense notion we all know inherently with the 
(psuedo?) logic of the investor collector 
(studying market values and interest trends in 
series) that we get all messed up and begin 
second guessing ourselves. Keep it pure, 
simple.

Just my 2 cents (if even worth that ;-)