My guess is that "they will make it up with volume".
Controlling large sums of money that can be moved around behind the scenes to the capture the best rates may provide the consumer a better deal than they can get with their more modest sums. BankSimple passes some of that back to their customer, and keeps the rest. My assumption is they will try to squeeze the margins down from the banks doing the heavy lifting, just as Walmart does with suppliers.
Controlling large sums of money that can be moved around behind the scenes to the capture the best rates may provide the consumer a better deal than they can get with their more modest sums. BankSimple passes some of that back to their customer, and keeps the rest. My assumption is they will try to squeeze the margins down from the banks doing the heavy lifting, just as Walmart does with suppliers.