It's illegal to sell breastmilk. At least in the United States. Since certain diseases and problems could occur in the transfer of the breastmilk, breastmilk is regulated. So if you go ahead with this plan then you will be doing it under the table, and no one really can tell you what they charge, etc.
Add: Of course breastmilk banks exist. They regulate the breastmilk, what can be found in them, what cannot be found in them. The requirements are typically very strict, they do a slow-pasteurization method, test for diseases, and test for a lot of other 'problems' in the milk that could hurt the babies. Most Milk banks are non-profit organizations, and the $2+an oz cost is to make up for all the regulations and testing that they have to do. The mothers typically get none of it in order to keep the costs down for the mothers who have to use the milk (typically they have to be given a prescription in order to even get the milk, the normal mother or infant isn't going to even be eligible to buy it, there are some milk banks that do allow others to buy though. It's more rare because there's such a demand for breastmilk and not much of a supply, so it gets triaged, given to the babies who need it most, like those with extreme allergies or those who are premature or very ill.)
Now the illegal question. I was under the impression that selling bodily fluids is illegal, whether it's blood or breastmilk. Donating it is legal. Selling it is not. But there's some question about whether it is or not, since breastmilk is 'milk' it should be regulated with cow and goat milk, but it isn't. Since it's body fluids, it should be regulated there, but there's question as to whether it is because it's also milk. I'm pretty sure in my state it's illegal.