For cost-reducing cards, the generally accepted wisdom is that in the absence of cost-related gainers or other shenanigans, you're not getting much out of them if you don't apply them to multiple Buys at a time. Bridge is a terminal Silver, Highway is a Peddler, even Bridge Troll is a Cutpurse with a Duration Copper. Quarry gets to be a Gold for Actions, which is pretty sweet, but that's about it.
So Ferry isn't great if you're only going to buy the cost-reduced card once. But then again, that seems slightly untrue, and I think the reason for it is that as an Event, its effect is immediate and persistent, both of which make it very strong. It essentially becomes like a Grand Treasury - an instant $2 that takes up no space in your deck and that you have access to as soon as you buy it, so the fact that it's priced the same as Silver is insane. I mean, ok, it's less insane once you realise that the $2 can only be spent on one card, and if you want to spend it on something different you have to buy it again. So given the number of times you get $3 and don't want a Silver, but wish you could have a magical disappearing Silver to guarantee a decent buy next turn, yeah I think it's got its use in those circumstances. Especially on those openings where you'd open $5/- if you could but $3/$4 is rubbish.
Then any time you *would* be buying the card multiple times, then it's the same as when you spend multiple buys on a turn with a standard cost reducer - you get that extra money on every purchase; the difference being that it works even when the buys are spread across multiple turns, something that only Bridge Troll can normally do.
So I think Ferry is a strong Event, and it looks like a really fun one to play with on a lot of different boards.