I'd say $3. Davio is right that +2 Cards soon nets more money than +$2 on average (though it peters out in the endgame). On the other hand, if you compare how the vanilla bonuses are valued on other cards, a consistent pattern of valuing +$ over (terminal) +Cards emerges. It took me a while to figure out why, but I think I may have: I think it's because cards with +$ are sufficient on its own to build up your deck, while cards with +Cards only get you in trouble if you aren't also buying other stuff.
Anyway, comparing and contrasting the value of (terminal) +Cards and with an equal number of +$:
At the $2 price tier:
* Moat's +2 Cards comes with a defense against attacks. But the only +$2 cards at the same price tier are a one-shot (Embargo) and a card whose other powers offer no clear additional advantage (Duchess).
At the $4 price tier:
* Smithy offers +3 Cards, but +$3 is too much even for a $5 card without complications (Mandarin) or unreliability (Harvest).
At the $5 price tier:
* +3 Cards is paired up with attacks (Torturer, Rabble, Margrave), while +$3 is, again, too much for a $5 card without complications or unreliability.
--
Now compare what a +$2 Moat would be up against in the $3 price tier: Chancellor, Woodcutter, Fortune Teller, Swindler, and Black Market. Swindler is the clear winner of this group, but do the special abilities of the other cards seem any stronger than Moat's reaction? Not to me they don't.