I agree that trying to make a price/rarity guide 
"seems" to be a good idee but is probably totaly 
impossible to do for several reasons :

- Prices are changing too quickly. Some serie
  may be worth far less (or far more) in just
  a few weeks.

- Some serie are over-priced compared to what
  people are ready to pay. So you can't find
  cheaper cels from those serie but nobody is
  buying. (So the price doesn't reflect the
  market value)

- Many collectors think that cels from some anime
  are rare and so expensive when in reality it's
  because nearly no dealer bother to put on the
  market cels from those anime.
  (A quick example : hentai cels. There isn't
  many on the net, not because they are rare
  but because most dealers don't want to bother
  with such cels)

- Prices from one cel to another can be totaly
  differant. Setting price per character (and
  not per anime) is really the minimum. But
  even so, often each cel need to be considered
  individualy because the value is based on a
  specific scene or clothes the character is
  wearing. (Nearly nobody ever bought cels from
  Neoranga except for the opening one that were
  very popular)

- Even two very similar cels can have very
  differant prices depending on when they were
  bought and from who.
  When I go to Japan to buy cels, often I buy
  two cels nearly exactly the same but one can
  be two time the price of the other. Just because
  they don't come from the same person/shop.


The only result from such a price guide will be :
If the dealer was planing to sell a cel at a lower
price than what is listed, he will increase his
prices.

If the dealer was planing to sell at a higher 
price, he probably won't lower it.

There will be even more people complaining because
someone is selling a cel for what they think is
too high a price.

Cheers,
Yann Stettler