I learned pricing on a by hour rate. Once you have some experience you should be able to estimate how long one picture takes to set up and render.  In a comic if multiple panels use the same set then it goes faster.
$50 an hour should be an absolute minimum in a big city unless you do it as a hobby or for the fame of your name on the comic.

I have concerns about you retaining rights to the characters if this is for his comic. If you are not partners he has left open a bad situation for competition.  If you are then you should be doing some of the work free, in expectation of profits from sales.

Get the final agreement in writing preferably by a lawyer. This seems the sort of situation that gonna bite someone right on the - ankle.