Why do we say "Every Man Has His Price"?

Meaning:

Anyone's support can be bought if you throw enough money (or alternate bargaining item) at the problem

Background:

An early reference can be found in William Coxe's Memoirs of Walpole written in 1798, which records Walpole (English Prime Minister) saying in 1745: "All those men have their price".

Personally I doubt that this was the first use of the term, or at least the concept, which I suspect dates back many thousands of years into man's pre-history.

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