True cost of “free”

Brokerage sweep gap = idle cash × (market APY − sweep APY). The broker is allowed to pay you less than market on uninvested cash; the difference becomes their revenue stream.

Brokerage spread cost = annual trades × per-trade drag. The per-trade estimate ($3–$5 default) is a synthesis of public PFOF disclosures and typical spreads on retail-traded stocks. Active traders, exotic instruments, and small-cap stocks all push this higher.

Credit-card interest= average carried balance × APR. If you pay in full every month, set the balance to zero — the card’s revenue from you drops to FX markup + interchange (the latter is paid by merchants, not you directly, though it ends up in retail prices regardless).

Checking foregone interest= average balance × alternative APY. A zero-rate checking account isn’t a fee in the visible sense, but the opportunity cost on $5,000 sitting idle at 4% market is $200/year. The bank gets to lend that cash out at higher rates and keep the spread.

What this doesn’t model.Mortgage / auto / HELOC interest spreads at the same institution (often where banks make their real margin); securities lending revenue (mostly invisible to retail); wire fees, paper-statement fees, “inactivity” fees on checking; the rebated portion of interchange on cash-back cards. The number above is conservative; real institution revenue per customer is generally higher.