You may pay too dear for your whistle.

Understand the thought

Meaning in plain English

A small or appealing object can cost more than its real value to you. Consider what you give up to obtain it, rather than letting desire alone decide what is worth paying.

What the words are saying

The whistle stands for an attractive acquisition, and paying too dear means giving more than the benefit reasonably warrants. The cost may include money, effort, time or another opportunity. The saying does not require a historical story to make that comparison useful, and no origin is asserted here. Its point is that eagerness can narrow attention to having the thing while obscuring the price. Value also depends on the person and the purpose. The useful question is whether the whole exchange remains worthwhile after the immediate excitement, not whether the object looks trivial to someone else.

Everyday example

A group decides not to spend its entire activity budget on an impressive decorative prop that would leave no resources for the event itself.

When to use it

Use it when desire for a particular acquisition is obscuring its full cost or effect on other priorities.

Agent-assisted editorial interpretation. This explains the words; it does not by itself verify their author or earliest origin.