I'm sure the Celtics have offered Rondo a maximum extension already. But unless I've misunderstood the CBA, here's a maximum extension if he were to sign one today:
2013-2014: $11,954,545 (current contract)
2014-2015: $12,909,091 (current contract)
2015-2016: $13,877,273 (extension year)
2016-2017: $14,825,455 (extension year)
2017-2018: FREE AGENT
So an extension would only give Rondo two more years of financial certainty. Last year a max salary would have ben $16.4 million. By the time Rondo is currently scheduled to his free agency, it will likely be between $17.5-$18 million, mean Rondo would leave around $4 million in salary per year on the table, plus 2-3 years of a contract. Rondo would be giving the Celtics an incredible home team discount if he signed that deal, especially considering the Celtics, at present, aren't terribly competitive. Rondo's a great guy, and my favorite player, but there's no way he should sign an extension.
My numbers might be off by a few hundred thousand in one direction or the other, but regardless Rondo would get paid at least 25% less than he'd command on the open market, and would add limited future financial certainty.
Danny Ainge would be foolish not to offer Rondo the most he could today, and to try again next year. But I don't think Rondo should accept it, and unlike with some stars, that's not an indication he won't re-sign in Boston. That's just how the CBA works for guys who have max or near-max value, but currently aren't paid that deal.