Assuming you're selling spells to other M-Us, and assuming you do that in the form of scrolls, the cost to make a scroll is the same for every spell. This varies from edition to edition; in OD&D, it's 100 GP/level. What an M-U would actually charge for that depends on supply and demand, as well as perceived risk to the scroll-maker, although probably double the cost would be a good bottom-line price.
If you are casting spells for others, you've got more potential clients, and no costs other than living expenses (unless you are using material components.) So again, it's all supply and demand and risk. Whether M-Us charge more for casting a spell than for a scroll of the spell, or the other way around, depends on the attitude:
- M-Us Stick Together: Going rate for spell casting is higher, because there are fewer M-Us than non-M-Us; you'll make more money from the rubes who have to pay again and again than from your colleagues, who only buy a spell once.
- M-Us Are Rivals: Going rate for spell casting is lower, because after all, prepping and casting a spell is faster than spending weeks making a scroll. If you sell a copy of your signature spell to another M-U, that's two people who can cast that spell instead of one; your business has just been cut in half.
A good guideline: one of the things an M-U is going to want to spend their earnings on is new spells, so figure out the cost of a typical scroll with a level equal to the max spell level an M-U can cast. How many spells of that level does the M-U plan on buying? Find the total planned outlay. Add the total expected living expenses, and other expenses, for one year.
Now: how many clients could that M-U expect in a year? Divide the expenses by the number of jobs in one year. That's the average minimum an M-U might charge. Actual charge would probably be twice that, perhaps modified up or down by spell level or riskiness of the job in question.
The third way to make money from spells, of course, is not to sell spells or spell-casting services, but to come up with a scheme or a product to sell and using magic to implement your plan. But these tend to be risky ventures; even if there's no legal risk, how much you make depends on whether your plan actually works. If you train wardogs and then Polymorph them into dragons for sale, you might make a lot of money -- unless one of them disobeys its master and goes on a rampage.