Americans are said to measure a person's ability by how much they earn.
Literal
Americans [topic-は], person [subject-が] earn money-amount [object-を] that person's ability [object-を] measure yardstick [to-に] do [quotative-と] said-are.
A complex sentence with nested clauses. 人が稼ぐ金額 is a relative clause: 'the amount a person earns.' それをその人の能力を測る尺度にする is 'to make that a yardstick for measuring that person's ability.' ~といわれている marks the whole thing as hearsay — 'it is said that.' Dense but grammatically rich.