Do you feel that you wasted lots of money on 
your collection?

I don't have any answers for the rest of what you 
were asking (though they're good questions all 
around), but I'd like to say I definitely don't 
feel I wasted my money on my DBZ cels.  DBZ was 
my first real anime, and I still have a soft spot 
for parts of it to this day. However, I think the 
other reason I don't regret spending what I did 
was that I've only gone over $50 on a DBZ cel 
three times in over three years of collecting 
^^.  Of those, two are staying in my collection 
forever, and the other one was sold for a good 
price.  I think the key in collecting DBZ--as it 
is in collecting any show that suddenly goes very 
expensive--is not to overspend.  Yes, they're all 
precious snowflakes, but that doesn't mean you 
have to drop a thousand dollars to get what you 
want ;P.

But yeah, it was a fad--just like Fushigi Yuugi 
is/was a fad--just like Gundam Wing is/was a fad--
just like Yuu Yuu Hakushou is/was a fad--just 
like Slayers is/was a fad.  There's no difference 
between DBZ and these others, other than the fact 
that the most sought-after characters are all 
steroid cases instead of bishy boys ^-~.