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.