Couple of things that come to mind... how many teams are out there that can realistically start a bid war for him? Mavs, Cavs, who else?
I also think Tiago has room to grow. Sometimes people gets stuck with the last impression, but he did improve considerably from last season to this one. He definitely has more work to do. The question is what do the Spurs think his ceiling is, and what's the limit they're willing to pay for that.
I think if the Spurs can get him back for around 7-8/season, it would be a good deal. I just suspect some teams might think he's worth much more.