I think Smart will re-sign here at a manageable rate (my guess, 4 Years/56M).
It's probably also telling that out of the IT/AB/MS trio that was set to hit 2018 FA, he kept Smart and Isaiah (not Bradley), and IT was gone the moment Irving became available.
Makes me think Ainge KNEW he wasn't keeping Bradley long term at a near-max rate and that he's both high on Smart and believes he can be kept at a "bargain rate".
Now unless some S&T is worked out involving Smart for some Anthony Davis trade next summer, I think Smart is here for the long run.