If you want to find out what the going rate in LA is, make some calls.  Pick up the phone book and call some graphic arts companies and ask them what their rates are for what you have been asked to do.  Get as much information as they are willing to give.  There is no set price.  Prices vary from city to city.  As it will hiring someone who has years of experience and credited work as opposed to hiring a relatively inexperienced artist.  A percentage of the back end sales sounds good but it really isn't all it's cracked up to be.  Back end usually means you get a cut after everyone else gets their cut.  Don't let this promise influence your price.  Consider it a bonus not part of main deal.  As for his paying you in increments, what exactly does that mean?  Does this client actually have a publisher for this comic or is he still trying to sell is idea to someone?  Does he even have an agent representing him?  Ask questions.  If he won't answer them, think long and hard before you jump into it.