Mystic seems pretty weak. +2$ is about the bare minimum I'd expect from a cost 5 card. Compared to Festival you're giving up a buy and an action for a chance at a card.
I agree that this is one of the more underwhelming DA cards (but something had to be, right?). However, it does seem balanced. If you get the card, you've got about a $7 effect. (The original draft of Grand Market was +1 Card, +1 Action, +$2 for price of $7 and seems about right there.) If you fail, you get a $3 effect. Basically, +1 Card on a non-terminal is the best a single vanilla bonus gets.
Anyway, the two possibilities average out to $5, so it seems like Mystic is well-worth it when you can skew the odds in your favor.
...And I think you can do that a lot more easily than you can with Wishing Well, whose unreliability might be clouding our judgment here. Sure, you can set Wishing Well up with Apothecary, Scout, etc, but the fact that it draws a card off the top prohibits you from using other combos, such as with Spy, Pearl Diver, Courtyard, Mandarin, Count, and, indeed, other Wishing Wells.
Mystic combos with everything Wishing Well combos with, plus all those others. And a chain of Mystics draws, at
worst, half the number of cards as Mystics, which would seem to make each one individually average out to a $5 effect. Possibly this is a good strategy in a heavily-trashed deck, unlike with Wishing Well, which -- even if it did combo with itself, which it doesn't -- wouldn't net you any money.
I might be talking myself into thinking it interesting after all.