I also think $5 and maybe put a cap on what you can play it as (otherwise, you most likely are getting a $4 Wharf after your first couple of them, and same for other powerful actions that are easily played every turn at some point when they are in the Kingdom, like KC, Governor, Apprentice, Hunting Party).
With Grand Market it can get INSANE.
You should really define better what "play as" means, because, if you play it as a Duration, what happens next turn? If you play it as Alchemist or Walled Village, can you topdeck it? If you play it as an action that has some below-the-line text (think Highway or Herbalist) that affects the game in moments other than when they are played? Probably copying Band of Misfists text is best here.