The opportunity cost of locked funds
A simple framework for comparing bank bonuses against the yield you give up by parking money in a low-rate account during a holding period.
What opportunity cost means here
Opportunity cost is the value of the next-best use of a resource you've committed elsewhere. For bank bonuses, the resource is cash and the next-best use is whatever yield that cash would have earned in the account where you'd otherwise keep it — typically a high-yield savings account at a competitive rate. If a bonus requires you to maintain $15,000 for 90 days at a bank paying a much lower APY than your benchmark HYSA, the yield you give up over those 90 days is a real cost of the bonus, even though no fee is assessed.
This is often the invisible line in a bonus calculation. The bonus is visible. The taxes are visible. The fees, if any, are visible. The yield you didn't earn somewhere else doesn't show up on any statement — but it's the difference between two paths in your actual cash position at the end of the holding period.
The math
The opportunity cost of locked funds is straightforward:
Opportunity Cost = Balance × (Benchmark APY − Bonus Bank APY) × (Days Held / 365)
Where:
- Balance is the amount of cash you actually have to hold at the bonus bank to qualify (the minimum to meet the requirement, not whatever you happen to put there).
- Benchmark APY is the rate you'd earn on the same money in your default high-yield account.
- Bonus Bank APY is the rate that account pays on the held balance.
- Days Held is the number of days you keep the balance at the bonus bank to satisfy the offer (often longer than the qualifying window if the account must remain open).
Net bonus value, after the opportunity cost, is:
Net Bonus = Bonus × (1 − Marginal Tax Rate) − Opportunity Cost − Fees − Time Cost
Tax is calculated on the bonus (not on the opportunity cost, which is a hypothetical not-earned amount). Time cost is your own estimate of hours spent times what your time is worth.
Illustrative hypothetical scenarios
All numbers below are hypothetical and chosen only to show the structure. They are not specific to any real offer.
Scenario A: small bonus, small balance, short hold
| Bonus | $300 |
|---|---|
| Required balance | $5,000 |
| Hold period | 120 days |
| Benchmark HYSA APY | 4.00% |
| Bonus-bank APY on the held balance | 0.05% |
| Rate spread | 3.95% |
| Opportunity cost = 5,000 × 0.0395 × (120/365) | ≈ $65 |
| Federal tax at 24% on bonus | $72 |
| Time cost (2 hours × $50/hr) | $100 |
| Net value | ≈ $63 |
A $300 headline becomes roughly $63 net after a fair accounting. Still positive, but smaller than the marketing suggests. Whether $63 is worth two hours of your time is your call.
Scenario B: larger bonus, larger required balance, longer hold
| Bonus | $700 |
|---|---|
| Required balance | $25,000 |
| Hold period | 180 days |
| Rate spread (benchmark minus bonus-bank) | 3.50% |
| Opportunity cost = 25,000 × 0.035 × (180/365) | ≈ $432 |
| Federal tax at 24% on bonus | $168 |
| Time cost (3 hours × $50/hr) | $150 |
| Net value | ≈ −$50 |
The $700 bonus loses money once you account for the foregone yield on a meaningful balance held for half a year. This is the case the headline can't tell you. The opportunity cost is where most "too good to be true" offers actually become not-worth-it.
Scenario C: bonus on funds you'd hold there anyway
A different shape: a checking bonus where the qualifying behavior is a direct deposit, not a held balance, and you keep your actual operating cash at a level that doesn't sit idle at the bonus bank. In this case the opportunity cost is near zero — you're paying yourself for behavior you were doing anyway. The full bonus minus tax minus time cost is the net.
This is why checking bonuses with direct-deposit-only requirements (no large minimum balance) often net out better than savings bonuses with balance-and-hold requirements, even at smaller headline numbers.
How the rate environment changes the calculation
Opportunity cost is rate-environment-dependent. In a near-zero-rate environment, the spread between a bonus bank and a benchmark is small, so the opportunity cost line is small and bonuses look better. In a high-rate environment (HYSAs paying 4–5%, the bonus bank paying 0–1%), the spread is large, the opportunity cost line is large, and any bonus requiring a meaningful held balance for a meaningful period loses value fast.
This is the reason bonus economics shift with monetary policy. The same offer that nets out well at one rate environment can net out badly at another. The framework — bonus minus tax minus opportunity cost minus fees minus time — is stable; the values plugged in are not.
When locked-fund bonuses still win
Locked-fund bonuses (those requiring a held balance, not just a direct deposit) can still be the right call when:
- The bonus is large in absolute terms and the spread to the benchmark is modest, so the opportunity cost is contained.
- You'd be holding the balance somewhere similar anyway — for example, a CD bonus on money you were planning to put in a CD regardless, with the bonus money on top of the rate you would have accepted.
- The "locked" balance is actually still accessible (some savings bonuses allow you to remove the qualifying balance after a short window — read terms carefully).
- The bonus is paid into a tax-sheltered account (an IRA), which improves the after-tax math substantially.
When they don't
Locked-fund bonuses lose against simpler alternatives when:
- The held balance is large enough that the opportunity cost line dominates the bonus.
- The rate environment is high (large spread).
- The hold period is long (months of foregone yield).
- The bonus posts to a taxable account at a high marginal rate.
- The bonus bank's tax-form treatment (1099-INT vs 1099-MISC) adds friction or surprise at filing time.
For the underlying mechanics of bank bonuses, see how bank bonuses work. For the tax piece, tax implications. For specific category context, savings bonuses and CD offers.