I was thinking last night about what this card would look like. I've come up with two different ideas, both treasures:
Investment - Treasure
When you play this, you may set this aside. If you do, at the start of Clean-up, trash this, and take 8 Coin tokens.
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When you buy this, you may overpay for it. If you do, take a Coin token per
overpaid.
ORInvestment - Treasure
When you play this, you may trash it. If you do, take 4 Coin tokens.
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When you buy this, you may overpay for it. If you do, take a Coin token per
overpaid.
I can't decide whether getting fewer Coin tokens to spend right away, or more Coin tokens that have to wait until your next turn are better.