Treasure - $3
$0
Trash a card from your hand or in play. Gain a card costing exactly $2 more than it.
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This costs $1 more for every 2 Coppers you have in play.
The idea started from thinking about alternative costs or buy restrictions, in the same vein as Peddler or Grand Market. I had the idea of "This costs $1 more for every Copper you have in play", but I realised that that becomes almost as bad as a straight Grand Market restriction for about 80% of cases. But $1 for every 2 Copper isn't so bad, and if its initial price is $3 then you can buy it for 5 Copper so it's still a potential opener.
Then I thought "So what could such a card actually do?" and I thought about how the cost-increaser is kind of neat for buying Farmlands, so I realised that it would be a nice self-synergy if it could do the remodelling itself - but that requires it to be a Treasure, to guarantee having Copper in play when you use it. Using it to trash cards in play is mostly so you can trash it to yourself, but you could also pull a trick like trashing a Copper in play to make it easier to buy another copy of it.
I'm not 100% attached to the specific wording of the effect - if it does a straight Remodel it can't really cost $3 even with the restriction, but it could probably do "up to $1 more" or "exactly $1 more" without too much difficulty.