If Zeller is extended this summer, I'd be pretty surprised if he got $10-12 million per year. Here's why:
Let's assume that Ainge is interested in keeping him long-term, and that Zeller is interested in staying. That's an easy assumption of they're talking about extensions. This assumption means that Ainge would make Zeller a qualifying offer, and does not intend on renouncing his bird rights next summer.
However, Ainge is focused on keeping clear as much cap space as possible. If he sees Zeller as someone remaining on the roster, that's a $6.5 million cap hold. If he waits until next summer to give Zeller a new contract, Zeller will count $6.5 million while Ainge is using his cap room, and then can go above that number to sign Zeller. So if Zeller's market value next summer starts at $11 million, and Ainge thinks that's fair, he can wait for a couple of weeks to officially sign Zeller to that amount, for an effective cap room savings of $4.5 million.
Accordingly, if Zeller signs this summer, Ainge will be getting enough of a discount to make cap sense for next summer. The question is if that discount matches up with the discount Zeller is willing to take for financial certainty. But I'd be very surprised for Ainge to pay more than $8.5 million in year 1 of a new deal -- in other words, sacrificing no more than about $2 million in future cap space. A new deal at $8.5 million next season could be for as much as 4 years, $37.8 mil. I'll round up and say I think that 4 years, $38 million is the largest such an extension goes for, an average value of $9.5 million. And I wouldn't be surprised if it's a bit less, more in the $34 million range.