Well, as for the current prices of those cels or
any other cels on a dealer's site (at least on
animechaos) are not set to inflate the price and
take profit of it. Nick once said that the profit
he gets with selling a cel is about 20% or even
less many times.

Anyway, as many other posters have said, trying to
get an objective price for a cel or even for any
kind of collectible is useless. Remember the times
when cels were just a subproduct of the making of
an anime/movie and consequently were destroyed.
That's why there are few or no cels of old series.

There are many factors to tell the price of a
collectible thing (not only cels), but the 2 most
important are:

1- The amount of people who collects that kind of
things. The more people the more competitivity to
get items, and consequently higher price.
2- The amount of supplies of that thing. The more
supplies it makes easier for collectors to access
the items, so that makes lower the prices.

So, at the end, these 2 factors (of course there
are others) determine the price of a cel.

According to this rule we can explain why dragon
ball cels are sometimes so expensive. It's true
that there are tons of cels from the series as
it's a long series with more than 200 episodes,
but it's also true that it has an insane high
number of collectors.

With X cels, well, the amount is low as it's a
movie. The number of collectors is also low if you
compare it with other series but still the few
number of available cels makes the prices to be
high as it's difficult for the few collectors to
get those few cels.