Lets analyze this.
Smart signed for $52 million contract over 4 years, which is $13 million a year.
What if Smart had taken the QO and his total earnings would have been the same?
Salary year 1: $6 million
Salary year 2-4: $15,3 million
Basically the Celtics signed him at a 46/3 contract, since he had no other offers. But there is risk involved for both parties.
For Smart there was a chance he could have gotten a better contract next year, but now he has certainty of a new contract. With the serious injury risks for basketball players I would easily estimate the risk at 25%. Which means that the guaranteed additional $46 million is roughly the same valuewise as possibly getting a 57/3 contract next summer ($19 millon year). Thus more or less an insurance pay of $11 million.
For the Celtics there was an opportunity to wait it out till next season. There's a little risk in giving Smart a new contract already. However a player as an individual should be more risk averse than a franchise, since consequences for them are much greater when things go south. Therefore I only assign a 10% risk for the Celtics. Which means that the guaranteed $46 million is equivalent to about giving a 51/3 contract next summer ($17 million a year).
Of course my risk estimates are a little arbitrary and factors like personal relationships, tax implications and potential earnings after 4 years aren't taken into account. But in a vacuum my conclusions are as follows:
For Marcus Smart: if Marcus expected to get a contract offer next summer (for a duration of 3+ years) with a salary of more than $19 million a year he shouldn't have signed and otherwise it was a good deal for him.
For Boston Celtics: if Ainge and his staff expected that Smart would be offered a contract over $17 million a year it was a good deal, but otherwise they overpaid.
Personally I think that Smart would have gotten around $12 million, which makes this a clear overpay. The uniqueness of Smart makes it bareable. For Smart it's a superb deal, although him and his agent might really believe that a team would pay him $20+ million a year next season.