I believe it is simple supply and demand, plus 
the commercial release of the Kenshin TV and OAV 
have probabily driven the prices up. The more 
people are interested in them the more we usually 
end up to having to pay, a lot of the kenshin 
cels on ebay are prime examples of this. I have 
found it is hard to win one without sniping for 
the cel in the last few seconds,which is why I 
don't win very many.
I would say the cels are not "better" then before 
but many more people enjoy kenshin, want them and 
seem willing to pay higher prices for them. I 
noticed the same sort of price trend with Tenchi 
cels after it was put on TV in the US. This seems 
to be slowing down a bit now though. 
Hope I could provide some insight.
-Ted