This seems most likely to be an error in the document: "median checking account balances at account creation are only around $3,000 ... At a 4% net interest margin, this account would only contribute about $120 in margin during its first year...." 4% of $3,000 is $12. I suspect both $350 and $120 are 10x too big. Alternatively the average checking account is 10x the median which seems unlikely.

> 4% of $3,000 is $12

no

Mainframes and lawyers aren't cheap.

The bigger cost is the branch and the teller, both of which scale less well.

If you think you can do it cheaper, build it. Save all of us money.