So here's some thinking:
1) Trader. Trader costs $4, and gives many Silvers on trash. That said, it won't give Silvers for Coppers + Curses, and you could (successfully) argue that getting Silvers all the time is a bad thing... nobody will trash a Province for 8 silvers unless they are playing Feodum, and even then. The difference in cost somewhat hinges on the difference between gain many (probably too many) Silvers to your deck vs. gaining one to your hand (Trading Post.)
2) Goons. Goons rightfully costs $6, and gives a lot of things. You could argue that removing the attack portion of Goons and nerfing its money down to $1 would make a card that is similar to goons but costs $3 or $4. So...
Nerfed Stoneworks -- Action -- $4
+1 Buy, +$1. When you buy a card when this is in play, gain +1VP.
I still think that this would be more cool as an on-gain effect, which is less powerful than an on-play, but if I also make it trigger on gains, then it is pretty damn powerful.
If I only make it trigger on buys, then it is weaker, but that would open up more design space... decisions.
I can trade the +$1 for a 'gain a silver' if I trash first, but only if I gain it to the discard pile, else we're in the $6-7 cost territory again.
Nerfedish Stoneworks -- Action -- $4
+1 Buy. Trash a card from your hand to gain a Silver.
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After you gain this card, get +1VP for each card you buy until the end of your turn.
This is closer to what I want, but still has two challenges: 1) you can turn Coppers and Curses into Silver. This is like the Trader, but with no reaction. It also doesn't do something when you trash Victory cards, which is pretty much something I was hoping to do to give it later-game longevity. So what about the following:
Longlasting Balanced? Stoneworks -- Action -- $4
+1 Buy. Trash a card from your hand to gain a Treasure costing less than it.
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After you gain this card, get +1VP for each card you buy until the end of your turn.
Now, this turns Coppers and Curses into nothing, but turns Estates into Coppers. Alternatively, in the late-game, this can turn the Actions you no longer want into Silvers or even Golds.
The negative side is that I've basically lost all reason for you to trash a Duchy or Province. For this reason, I am proposing:
Proposed possibly moderately OP Stoneworks -- Action --$4
+1 Buy. Trash a card from your hand to gain a Treasure costing less than it. If you trashed a Victory card, get +1VP for each card you buy until the end of your turn.
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After you gain this card, get +1VP for each card you buy until the end of your turn.
I trust that I will test this, love it, and then realize it's too much text to put on one card.
What is your feedback, especially on cost? Which version feels most like $4?