My gut reaction is that would be too powerful. Even without thinning that's now guaranteed terminal Gold at worst, very often terminal +$4, and if you can trash some coppers it'll be +$5 and above frequently.
I'm not sure whether that is too powerful. A hypothetical terminal +$4 would be strong (and boring) at $6, but I think not too strong. I think BM-(this card) wouldn't fare that well, and engines usually want something besides plain terminal money.
Similarly Adventurer searching for 3 cards would be (very) strong, but maybe not too strong:
Big Money doesn't trash, so it would give ~4-5 money each play. That's strong, but I doubt it would beat BM-Smithy, because $6 is a high price for BM-X. Another comparison: I think that when playing Big Money-Hunting Grounds, the HG will draw on average 3 treasure cards when played (a little more when you just started greening, less at the end of the game), so this would be similar in strength.
For engines, this card is almost strictly worse than Hunting Grounds. Sure, you can't draw actions dead, but engines usually want to draw their actions before their treasures.
In slogs this would certainly be very strong. But $6 is quite a lot for slogs, I think it wouldn't be a problem. And this card is way too expensive for rushes.
So on paper it looks actually not that bad. I think it's worth testing.