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Is Bank of America’s plan to charge customers who use their debit card for payments $5 a month next year reasonable in terms of what such transactions cost the bank?

That depends on the math and the customer’s spending habits.  If a customer uses a debit card 25 times a month at Bank of America and each transaction nets the bank a 21-cent fee, that’s $4.75.  Under the old rules, before the decrease in debit card fees from an average of 44 cents each went into effect  Oct. 1 under the Dodd-Frank Act’s Durbin Amendment, the bank would have netted $10.

Maria Aspan, consumer finance editor with the newspaper American Banker, notes that a customer making 25 debit card transactions each month would indeed cause the bank to lose $5 it would have made before the Durbin Amendment.

A banking survey from the Federal Reserve from June found that the average, or mean, per respondent of total processing costs for all types of debit card transactions, including those on prepaid cards, was 17 cents. That 17 cents does not include fraud settlement or other services.

If a bank’s overhead costs are 17 cents a transaction on average, and they are capped in charging customers a maximum of 24 cents per purchase, they make a profit of 7 cents for each transaction.

Apparently, that 7 cents isn’t enough for Bank of America — and several other big banks are experimenting with $3 monthly fees in some markets.

For heavy debit card users, the $5 at least makes some sense. Customers who only use their debit cards a few times a month to make purchases may find the fee harder to swallow.

But Aspan cautioned that these fee estimates may not apply to Bank of America, which is the largest bank in the country.

“Because the bank is so big, its overhead costs are likely to be higher than the average as calculated by the Fed, so I doubt it’s making quite that much profit per customer,” she said.