I do simillar work, so perhaps I can give you some perspective.  First, the answers to these questions will be very telling:

1:  How did he find you?  If you met him in person and this came about during conversation, fine, but check HIS references.  Has he done this kind of work before?  If so with who, and talk to them.  Especially any other artist he's worked with.    If he contacted you by seeing your website/stuff on 'rosity/email.... and you've never actually met this person..... beware.

2:  I am a pro, so here i'll help you incredibly.  Once you set a price GET A WRITTEN CONTRACT.  Thats right.  If you don't, i bet three million dollars you wont get paid one red cent.  Find out exactly what percentage he will pay you, make sure thats in writing on a contract.  No contract?  Don't work for the guy.  Even a self typed contractual letter by him and hand signed is good enough, you can use that in court should he shirk you.

3.  Ask yourself, who markets a comic book....... who doesn't actually make the art or already have a collaborator?  If he is just writing a story, and you are doing all the art, your collaboration (thats right, collaboration) should be a 50 / 50 split of profits.  Both names on it clearly (ie story by xxx artwork by xxx).  Its the art that sells a comic book.

4:  Roughly, illustrators for comics (that are just starting out) roughly ask for 100 dollars a print page, which could be numerous cells.  Once you are known and out there, you can raise that...... a LOT.    Hit this guy (contract term here lol) for the high end amount first, and see how bad he recoils.  If you think 100 dollars a page sounds good to you, ask for it.  Work from there, he'll whittle you down no doubt.   Before you even speak to him, know what your highest is you ask for off the bat, and your lowest (say 50 per print page)..... and dont accept anything lower.

You might loose a job not going lower, but you arent working for free either.

5:  DEMAND the first or second print, numbered by the way, completely free of charge.  If somehow it flies off shelves and becomes some cult or mainstream goodie, thats serious money in the bank.

6:  You read the story board, do you think it will sell?    Don't ask "well its work, I should do it".... ask yourself, "Its a lot of work to invest, is this worth investing in?"  If the answer's yes, go for it.