Lets see... you could trash 5 Coppers, and open with XXX for sure. During each of your next turns, you will be able to get either your starting hand (which will get you another XXX) or draw an XXX to basically add +$1 to your starting hand (but you should get more XXXs first). Sounds a bit too dominant.
What about letting them trash only up to X Coppers, maybe 3?
It would suck if that was the right play. Which it could be. I don't like putting a restriction in the setup though. Maybe the top half needs to change so that it doesn't stack anymore.
... which leads back to the problem of, what kind of top does the job. I actually didn't intend to write such a long response, but this got me thinking about all possible combinations of vanilla-only cards that could cost 2$. that's interesting anyway, so let's do it (all effects that aren't immediately terrible in red):
1 or 2 different ones: all < pawn
2 of the same: +2 cards (< moat), +2 actions (< squire), +2 coins (= duchess), +2 buys (< squire)
4 or more is probably either silly or too strong. though I did try +3 actions, +1 buy once. but let's skip that one.
3 different ones: action/coin/buy (< CSM), card/action/buy (< Market Square), card/action/coin (=Peddler),
card/coin/buy (terminal market)and 3 not all different:+3 cards (= smithy)
+3 actions (tried that one with +buy, didn't like it)
+3 buys (silly)
+3$ (too strong)
+2 cards, +1 action (= lab)
+2 cards, +1$
+2 cards, +1 buy+2 actions, +1 card (= village)
+2 actions, +1$ (< squire)
+2 actions, +1 buy (boring and weak)
+2$, +1 card+2$, +1 action (= silver)
+2$, +1 buy (= woodcutter)
+2 buys, +1 card (eehhh)
+2 buys, +1 action+2 buys, +1$ (< squire)
so these are left:
+1 card, +1$, +1 buy
+2 cards, +1$
+2$, +1 card+2 cards, +1 buy
+2 buys, +1 action
+2 buys, +1 action doesn't really make sense on this card, and it's also mphf. I'm sure you would buy it sometimes, but it doesn't feel right. same more or less goes for +2 cards, +1 buy. maybe if it was a different concept.
so, that only leaves the two coin/card combinations, the terminal market, maaaybe things with a 4+ bonus, and non-vanilla effects. right now the terminal market is the only thing that feels like it's worth a try.