The series that are decreasing in popularity will 
get cheaper, like Orange Road.  KOR cels went up 
a lot during the mid 90s, but the prices now have 
dropped back down when the demand is low.
For the series that continue to get the TV 
exposure and popularity like Sailor Moon or 
Dragonball, prices may continue to go up for more 
longer.  Those series have been out in Japan for 
many years, the older fans who used to be kids 
are now grew up and got a job in the real world, 
that's why the prices have pushed up so high now 
because these older fans can finally afford to 
pay the prices.  As long as there're people 
collecting them, the prices won't drop much.