Quick note, I also made a Werewolf card in the past: "+3$, each other player reveals a Silver or gains a Curse", for 5$. Seemed to be fine. As you can see, it followed the same "attack that doesn't like Silver" theme.
About yours, I think a board with a Peddler that optionally trashes has limited use for Silvers. Likely, they will be used as a stepping stone to get to the card, and after that might get trashed. In fact, I think this Werewolf's own attack is harming the card much more, as it makes you discard junk you could've trashed. Whoever gets it first will probably be able to slow down the other players a lot. As an aside, I would probably go with "when you play a Silver, -1$" so people don't have to ask whether it can produce less than 0$.
Edit: On second thought, it's kind of clever how this card's price makes Silver a great tool to get the card, while clearly it hates Silver once in the deck (also because it's a cantrip, btw). So this card might start with a buildup phase, where Silver is useful, then you get a Werewolf and clean out that Silver, and ultimately you get a thin Peddler engine. The only problem I have is how - much like Warrior - this card itself hinders other players to get it. Sure, players can try to get a deck where discarding down to 3 cards still lets you buy a 6$ - but on quite a few boards this means getting even more Silvers, making the Werewolf you intend to get even worse. Probably getting some sort of Death Cart or Mining Village is the best strategy here, but often you won't have that. It does indeed create quite a few considerations you have to go through, I think...