Grant makes the MLE (13.25 average), that seems about right for a 6th or 7th man which is what he is. Now he probably doesn't have much trade value because he is a 6th or 7th man in the first year of the new contract, but I wouldn't call him over paid either.
I guess we will see but with the new CBA restrictions, I think full non-taxpayer MLEs are going to need to be used for better than 7th/8th man players. Each team is going to have 2-3 max or super max contracts, a couple of MLE contracts, and then all min or rookie contracts. Even last season, very few teams could have offered Grant that contract. The Celtics could have matched but only non-taxpaying teams could have made the offer.
A team can choose to resign a player like Grant to this amount via Bird rules, independent of the MLE, but in this case, the Celtics chose not to. I think most teams would chose not to sign a 7th/8th guy for the equivalent of the NTP MLE. Dallas thought he was going to be a 3rd/4th level player. Had they understood he was really a 7th/8th player, I do not believe they make that offer.
Currently, the Celtics pay their 6th, 7th, and 8th guys $10M, $4M (soon to bump up to more like $7M), and $2M. In coming seasons, a team like the Celtics may not even be able to pay that much. Signing Grant also hard-capped Dallas as they used the non-tax MLE. So to overpay and end up hard-capped, is not something smart teams are going to do for a marginal bench player.