The volume or duration at which two options cost the same, so that the cheaper of the two changes on either side of it.
A break-even point is found by dividing the fixed cost carried by one option by the amount that option saves in each unit or each period. Below the resulting figure the option with no fixed cost is cheaper; above it the option carrying the fixed cost is. The arithmetic is trivial and the inputs are not, because the fixed cost is usually an estimate while the recurring cost is usually a quoted price, so an error on one side alone moves the crossing a long way.
See also
A comparison of the whole life cost and the wider consequences of producing something in house against acquiring it from the market.
The full cost of a thing across its life, covering acquisition, running, change, support and eventual retirement, rather than the purchase price alone.
Where this comes up
A delivery manager does not need to know contract law. They need to notice the moment a delivery problem becomes a contractual one, because after that moment informal handling starts costing money.
A make or buy analysis compares the whole life cost of building something against buying it, and finds the point where the cheaper option changes. Capability, control and risk usually decide it.