Yeah, this is a pretty great card design, but it does need to be $4. I think it’s better for it to give out Coppers rather than curses. It’s nice for it to be slightly weaker and more newbie friendly, and it’s less damaging to certain alt VP slogs that way. I would use LFN’s wording and not care that it’s weaker in Council Room games or whatever; it’s already very solid.
Based on the discussion I agree with $4. I mostly chose Copper over Curse because I want the 'discard something useful' vs. 'gain something bad'-ratio to be closer together - people are more likely to opt for gaining a Copper than a Curse. With Torturer this is achieved by the effect stacking.
One benefit of Curses (and Ruins) is that the size of the pile is catered to the number of players. All existing junkers that hand out Coppers do so sparingly, have conditions for not obtaining them, or help get rid of them. Mountebank slows down or stops as people get more Curses. Noble Brigand, Hex attacks, Jester, maybe Messenger, don't always do it. Ambassador gets rid of two before junking. Some cards have Copper/Curse as a penalty, but this is the player's own decision. All this puts a limit on how much someone's deck can be junked, perhaps because junking beyond some point makes the game too slow and unfun. I think this is why there are no unconditional Copper junkers - the pile is simply too large.
I think I can get away with Coppers because the decision of getting junked is in the attackee's hands, so you don't necessarily need the balance of a limited Curse pile. (Edge case: a hand of all Victory cards, but that barely happens, and as you get more Coppers you get less of these hands)
There's also "each other player discards down to four cards in hand, then gains a Copper if the top card of her discard pile is a Victory card."
The Councilroom engines won't be dissuaded from playing the card.
I think I would only use the top card of one's discard pile if other cards in a set use it. It does make Tunnel a good defense against this, though.