What it would cost to hire the work back
Institutions never treat an unpaid obligation as free — they price it at replacement cost, the market rate to have someone else do the same job. Applied to a household, that is the pre-tax salary a returning second earner would have to earn just to buy back the childcare, cooking, cleaning, and driving a stay-at-home spouse already provides.
The headline is not a sentimental number — it is an underwriting one. The labor of a stay-at-home spouse is a real input with a real market price, and that price is paid in pre-tax dollars: a second salary is taxed before any of it reaches the daycare bill, so the salary required to break even is always larger than the costs themselves.
That is why a second income so often disappoints. Slide the prospective salary against the replacement cost and watch the flip: below the line, the job pays for the replacements and a little more; above it, the new wage is consumed entirely by the services it forces you to outsource, and the household is no further ahead for the hours spent away.
Assumptions, all adjustable above: replacement costs are quoted at retail monthly market rates and grossed up at one marginal-plus-payroll tax wedge; service sales tax is shown but not added to the salary requirement; the prospective salary is a framing bar, not a modeled cash-flow. A teaching instrument, not financial advice.