I think it has to do with the American Production Cel Market
Think about ti this way:  I bought a Plucky Duck cel from
Tiny Toons years ago for $450.  It's not uncommon for Cels
from american productions to go for up to $1000 or more.

I think that the issue is that when a show appears on American
TV it opens itself up to more collectors that are used to
these prices, and so they continue to pay them.

Just wait until August, it'll happen to Tenchi, too.