Idiom or phrase to describe a short-lived reward

A common expression is the extragrammatical but transparent idiom, Easy come, easy go.

easy come, easy go [informal]

said when something, especially money, is easily gotten and then soon spent or lost

[Cambridge Dictionary]

• A: I lost the fifty dollars I won at the casino really quickly.

• B: Well, easy come, easy go.

[LanguageCouncils.sg]

(The idiom is also often used of the character of someone content to live in the way implied.)

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Another, slightly less transparent metaphor is Riches have wings / Money has wings.

Riches have wings:

... Money can disappear easily. Money is like a bird with wings: it can fly away if you are not careful.

[EnglishClub]

The source of this metaphor / adage is the Bible:

Proverbs 23:5 New International Version; BibleHub.com

• Cast but a glance at riches, and they are gone, for they will surely sprout wings and fly off to the sky like an eagle.

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And there are various translations of Proverbs 13:11 [again, BibleHub]

• The more easily you get your wealth, the sooner you will lose it. The harder it is to earn, the more you will have. [Good News Translation]
• Wealth gained by dishonesty will be diminished, But he who gathers by labor will increase. [New King James Version]
• Wealth gained hastily will dwindle, but whoever gathers little by little will increase it. [English Standard Version]
• Wealth from get-rich-quick schemes quickly disappears; wealth from hard work grows over time. [New Living Translation]

Pick whichever version best fits your purpose; preachers do.