That's not correct : series like AMG or Lodoss
are OVA and not TV. So they are of higher quality.
Those series are also older and much shorter so
there is far less cels. (It's like if there was
only 5 or 13 episodes of DragonBall in all).

Above all that, TOEI released very large 
quantities of cels from DragonBall. (in the
same way as AIC for El Hazard/Tenchi TV series)
Which was not the case (and by far) for series
like AMG or Lodoss.

When you think that some DBZ cels are expensive
because they are rare, you are wrong : those
cels are fairly common and until not long ago,
they were pretty cheap.
Now, they are still quite easy to find (so they
are not rare) but they are expensive because
some people started to offer very high prices
for those cels. Naturaly, when dealers/collectors
in Japan saw what kind of prices some people
are willing to pay, they raised their own prices.
(It's probable that Yahoo auctions were 
responsible in great part for that)

It has nothing to do with such cels becoming
rarer or having more people wanting to buy them.

Yes, they are rarer on-line now : not because
there is less cels available but because 
non-japanese dealers or not willing to buy them
at such high prices.

Cheers,
Yann Stettler