Prices are not raising as much as many people
seems to believe. It's only that when we, 
old-timer, were buying Tenchi cels (for example),
nobody wanted them because it was a new anime.
People wanted Urusei Yatsura, Maison Ikkoku,
KOR and complained that the prices on those 
anime were raising.

A serie like Tenchi is no longer a new anime.
Cels from it are no longer put on the market
by large quantity. And it's now a popular anime.
But there is plenty of new anime with cels
as cheap as Tenchi's were. Not many people
buy them. And in a few years, a new generation
of collectors will complain that they became
much more expensive... (Ok, perhaps they won't
have much the choice : They may not be cels from
new anime then. If all studios switch to CG...).

It's a lot like stock market : If you always
buy what everybody want, it will always be
expensive.