The idea of a $2 cost Attack is interesting--it's tricky to design since the attack and benefit need to be mild enough to fit the low cost, but not so mild that the attack effectively does nothing (i.e. imagine an attack even milder than Fortune Teller). My solution is to make the card require multiple copies to actually attack and to get stronger with more copies, so while it's cheap, it still requires an investment. Note: I have not tested this at all, so it could be a nothing card.
By itself, a single Knave is just a Copper, not a great deal for $2. But with two of them in play, you get a nice discard attack, and each one after that Curses your opponent too. Knave is non-terminal so you can easily play several of them, but the tricky part will be when to gain them and how to line them up to start attacking. It's a pretty bad opener, and the benefit to your deck apart from the Attack is very slight. It will take a few turns to start attacking consistently, and once it does, the discarding and cursing will make it harder for players to keep lining them up. It's possible that unlimited cursing after three in play might get a bit oppressive, but I figured that getting to a point where you play more than three of these consistently will usually take a while. I toyed with the idea of giving them a +Buy to make it easier to get more of them, but +1 Action +$1 seemed stronger on average than +1 Action +1 Buy. Not sure though. Overpaying to get extra copies or Forum-style "When you buy this, +1 Buy" are also options, but that's a lot of extra text on an already wordy card.
Thoughts? What are other ways to make a $2 cost attack work?