A rundown:
+1 Card, +$1 - As has been rightly pointed out, this is strictly inferior to Pawn. It's probably not worth having even if it had a $0 or $1 price tag, unless it was truly the best terminal on the table and you have some extra buys.
+2 Cards, +$1 - For this, and for all combinations of 3 such bonuses, you have to compare Smithy (costing $4) with a terminal Gold (balanced but boring at $5). So this would be slightly stronger than Smithy. Probably still costable at $4, but insufficiently distinct from it and other cards as to be worth printing.
+1 Cards, +$2 - Stronger still. (I should emphasize that this is not ALWAYS stronger than Smithy and would in fact be weaker in a lot of decks. But if you look at the official cards collectively, +$ seems to be valued slightly more than terminal +Cards are.) This one might have to go up to $5, but it would be a weak $5.
+3 Cards, +$1 - A $6 card, given the difference between Village and Bazaar (both of which seem exactly middle-of-the-pack in their respective cost tiers). Again, though, this is a case where being balanced doesn't make it interesting. And in any case, if you need a Smithy-type card in your deck, you probably don't want to wait until you've got $7 to spend to get it.
+2 Cards, +$2 - I agree that this is the most interesting combination. And it might be workable at $6, as many have said. But I think there's an excellent chance it really needs to be at $7. Compared to Gold, it just seems so much better, with "only" the fact that it consumes an action as a drawback. I mean, figure that with an average card value of $1, this effectively amounts to a terminal +$4. If we accept that a terminal +$3 is a $5 card, then a terminal +$4 is surely going to warrant larger price increase than $1.
+1 Card, +$3 - Here, the $7 cost is even more convincing.