If you've got a dollar and you spend 29 cents on a loaf of bread, you've got 71 cents left; But if you've got seventeen grand and you spend 29 cents on a loaf of bread, you've still got seventeen grand. There's a math lesson for you.

Framed as a “math lesson,” the joke is really about perspective and relative impact. A small expense can meaningfully reduce a small budget, while the same expense is negligible to someone wealthy—so the arithmetic is correct but the lived consequence is radically different. By using an everyday purchase (a loaf of bread) against a conspicuously large sum (“seventeen grand”), Martin satirizes simplistic financial reasoning and the way wealth insulates people from ordinary tradeoffs. The punch line also pokes fun at didactic advice: the “lesson” is obvious, yet it exposes an uncomfortable truth about inequality and how money changes what counts as a sacrifice.