I think Adventurer would be fine at $5, but it's also fine at $6. It's just one of those narrow cards, like Counting House, that is rare to find a use for, but fun when you do.
The ideal Adventurer board (for me) has Moneylender as the only trasher, and perhaps Cursers flying around. For an extra bonus, some sort of non-terminal +Buy like Market or, better yet, Worker's Village. The point is, if you can trash your Coppers, Adventurer becomes awesome, worth anywhere from +$4 to +$6 (or even more, in Platinum games). Moreover, it becomes important, since if you have a lot of non-Copper junk in your hand, Adventurer might be the only good way to get a lot of money together in one place and thereby afford Provinces (or Colonies).
So yes, I do think it works at $5, but on the right board it is a bargain at $6. I don't think that's true of, say, Harvest. Anyway, that right Adventurer board might not come around very often, but when it does it's pretty sweet.