I think you could make the argument that Smart has the same upside as Lance Stephenson had. At his best, Stephenson was a good defensive player and 4th option on offense, averaging 14, 7, and 5. Stephenson always had an edge and good intangibles.
At that level of production, the best contract he could get was was a 9 million dollar contract from the Hornets when the salary cap was 63 million. Comparatively, that would be 15 million a year under the current salary cap.
Comparing the two, Stephenson has figured out how to get his shots to go down, although he still isn't as good of an offensive player as his basic stats indicate. However, Smart seems to have great intangibles and is a bit better in the pick-and-roll.
I don't think Smart's value goes higher than 15 million a year, especially considering how little money is available next off-season. At 26, Stephenson could not earn more than a 4 million dollar contract with the Pacers. At 24, what will Smart be able to earn considering his woeful shooting?
In other words, I think the most Smart can hope for is 15 million, and that dollar amount comes down from there considering the competition to sign him will be small and his shooting has lowered his value.
In the end, I think we end up around 45-50 million over 5 years. Expressing that security to him in a long-term contract, along with getting him as the proper cog in the offense as the 4th-5th option, will likely raise his shooting percentages during that time. By the end of that contract, I think he will be one of the best non-rookie contracts in the NBA.