I like it being able to trash copper, to deal with the fact that this will be adding copper.
"You may trash up to two copper from your hand, +1$ per copper you trash this way" seems reasonable. It doesn't help you thin out the other cards in your deck, but stops you from being overwhelmed by copper.
Food for thought:
• Getting junked with Copper is way less harsh than getting junked with Curses.
• This attack can miss and doesn't stack (unless you play something else that makes them discard the Copper they gained on their deck).
• No published card that hands out Curses gives you a way to trash Curses (except Ambassador, but then you need to gain Curses in some other way).
This is probably less harsh than any published junker. So I don't feel a really pressing need to make it also trash Copper. That's what the other cards on the board are for. When there's a good way to get rid of Copper, this attack becomes worse. When there isn't, it becomes better. Making it a solution to itself seems unnecessary.
What if you gained a card costing up to $X on top of your deck? That's a little more interesting than just gaining a gold.
I was thinking about this more. It thins your deck and adds cards you might want for an engine. (How strong depends on the price point you give it and the value of X)
How does it thin your deck? Are you talking about combining Copper trashing with gaining a card on top of your deck and the attack? That's a bit much I think.
Concerning just gaining a card costing up to $X on your deck, it seems like it could work, but let's examine it. In general, gaining a $5 card of your choice is way, way better than gaining a Gold. On some boards you'd rather have the Gold, but on most you'll take $5 Actions early and Duchies later.
So if X=5, this card is nuts. Altar is already a good $6 card, and even assuming that this attack is roughly equivalent to trashing a card (who knows if it is, but bear with me), the fact that the $5 card goes on top of your deck is insane. I mean, it might work if it cost $7 or more, but I don't really want to have any cards costing higher than $6 in the set.
If X=4, we have Armory with a bonus. So probably it would cost $5. I don't know if I find that terribly compelling, but it's a legit card for sure.
X≤3 isn't usually going to be popular. Then you have a weak Attack combined with a weak gainer. Probably not a crowd-pleaser. Even at $2, I don't know that I'd buy it in most games. Well, maybe I'm underestimating it. I guess there are a fair number of really cheap cards you want a bunch of, and it does put them on your deck. Hmm… Maybe I should try it out.
The reason I suggested Gold is that I feel it fits the theme of the card; it's analogous to the Coppers you're making your opponents gain on their decks. I think it's nice to have some cards that gain Gold. On the other hand, Soothsayer is another card that junks and gains Gold.
I'll think about it more and I might test several versions. Do you guys like the version that gains cards costing up to $3 or up to $4?