I think it's easy to see a FA Marcus Smart (current version, if he improves, all bets are off) netting a contract around what James Johnson and Igoudala got this year. Basically, 4/60. It's harder to see him in the $20/year range of guys like Hill, Teague, Ibaka, and Gallinari.
But, he's not a free agent, he's restricted. We've seen this off-season that being restricted has a dramatic impact on submax players. Smart is not a max player. His problem is, any team that values him at 4/60 would be foolish to offer it because the Celtics would match. So, you need to find a team that overvalues him and has cap space to make an offer worthwhile for them. Only one restricted FA found that team this year. (Maybe 2 if you count Porter.)
If I'm Ainge, I'm probably targeting 4/48 or so as a fair extension offer based on my sense of the market. I'll still be thrilled, though, if we extend him for anything up to 4/60.