I find the trouble with opening Mint / x is that you end up with a 7-card deck, out of which 4 are useless Estates and Mint, and thus a pretty good probability of not getting enough to buy Silver for, actually, a surprisingly long time.
In your case, Haven is just the right thing, since you can get it with $2 and it's great in this kind of deck. If there wasn't a good $2 buy, you could easily end up drawing $2 on turns 3 and 4 and 5... With Mint/Pawn, you only effectively have $3 total, so it's even worse.
P.S. Haven is not one of the worst, it's one of the best.