Depends on how long they want it for and what they're going to do with it. If you're actually selling all your rights for all perpetuity then you should charge a lot, otherwise you should license the work to the person for a limited time to make it cheaper for them. What you charge is entirely dependent on the size of the company, what business they are in (a magazine publisher has a much smaller budget than even a small movie studio or advertising company, for example), how long they want it for, how many different kinds of rights they want (the right to display it on the Net only or the right to display it online and put it on t-shirts?) and so on and so on.
I think first off I'd ask them what their budget is and what rights they are looking for. Then go from there.
Usually it depends on how much money you expect that they'll make off of using your work. If the person is a small company or self employed then they likely will not be making as much off of your work as a larger company would. Charging an additional amount for commercial use is usually called a royalty fee (which is a cost that is calculated in addition to the cost of creating the initial artwork), and it covers an amount that you think is fair in exchange for the continued use of your artwork to promote their business.