The rising price on cel is not mainly related to 
CartoonNetwork.  Anime has been widely exposing 
to the US in the past 10 years from commerical 
and fansub videos, then in the past few years, TV 
network starts to air more anime to accerlate the 
popularity of anime. 
Let's take for an example, I bought a Deedlit cel 
of Lodoss War in Japan around 10 years ago for 
less than $20, now that cel worths over $1000.  
Record of Lodoss War never aired on TV, but the 
video popularity has attracted tons of fans into 
this series.