Recently I was commissioned to create a cartoony character for a software that is going to appear on the interface, software, website and (hopefully) on promotional materials. It is done now, the final version has been agreed on. We have already discussed creating different versions of the character for different purposes (viral ads, animations, etc) so we're talking different outfits, poses etc.

It is my first commission in this area of art and as much as I love working on this assignment I would like to know it is all worth the effort.
I think I could charge for the finalized main concept now and later on as per each new alteration/illustration. I do not, however, know how much could I ask without going overboard cause I realize that this is a great chance for me to get noticed on the market/get a nice point for work experience into my resume especially if the software catches on and goes big. So it is not only the money at stake that I am considering.

I am hoping that one of you have been there and could give me a clue on what are the usual procedures, fees etc.

I would really appreciate a word of wisdom.