129 panels is only 1 quite small issue of a comic (12-15 pages.) Depending on the storyline, it could take months to set up a Poser scene for each panel, render, postwork, etc...

It's interesting that he's offered you a percentage of sales. This makes me think that he'd be better off looking for an artist he can collaborate with on a project, on a percentage-only basis. "If the comic book is successful" - has he even got a buyer?

Paying by increments - this means you could put hours of work into developing characters, only to find you use them once and that's it...

What with me being a bit of a comics fan, I'd ask to take a look at his scripts first, and if I liked them and thought it would be commercially successful, I'd take it on at a fairly low rate with percentage. (As others have suggested, call around and get estimates on the going rate in the  area.) But I'd make sure I mocked out some of the panels first to get an idea on how long each one will take.

Bear in mind if this is for print media you're going to need to render at high res - that could potentially double or triple your worktime.

Just some more thoughts to ponder...

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"you are terrifying
and strange and beautiful
something not everyone knows how to love." - Warsan Shire