I think you're missing the market effects of having a Wal-Mart. If Walmart moves in and offers MRIs at half the price of everyone else, then everyone else will

I think you're missing the market effects of having a Wal-Mart. If Walmart moves in and offers MRIs at half the price of everyone else, then everyone else will have to offer better prices to compete. Someone who might not have been able to get an MRI at all will now have the Walmart option, and you'll still benefit from lower prices at your "premium" option.