Okay, how about:
Cursed Idol
$4 - Treasure
$3
When you play this, trash another Treasure in play, and gain a Curse, putting it on top of your deck.
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You may not play this unless you have at least one other Treasure in play.
You could just give cash for trash instead having a restriction.
Cursed Idol$4 - Treasure
When you play this, gain a Curse.
Trash another Treasure you have in play.
If it is not a Copper, this is worth $3.
Looks a bit like Counterfeit, so what about a derivate of LFNs version:
Cursed Idol$4 - Treasure
Worth $3
When you play this, gain a Curse, putting it on top of your deck.
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When you gain this, trash a card you have in play. If it doesn't cost more than $2, trash this.