I'd give him a "Westbrook-like deal", at around 3 years/87M (29M/Year), with a player option the third year, like Westbrook has it.
Now if people still think he's not worth that, then honestly, idk what to say.
He averaged 29/6, was an MVP candidate, now a 2x all-star, and led this team to a #1 seed in the ECF and played injured for the final part of the Wiz series + beginning of Cavs series. Also had a spectacular postseason considering the circumstances (the tragedy he dealt with).
What he needs is another few good pieces around him (Fultz + Hayward/Griffin? + Zizic), because lets be real, AB, Crowder, Horford are great players but NOT legitimate, consistent scoring threats, which Isaiah definitely is.
How far do you think Washington is going without Beal, or Toronto without one of DeRozan/Lowry? The C's meanwhile made an ECF with just one Isaiah, and no other true scorer.
Also, it won't matter how much better this defense is if no one can score, as we saw in Games 4-5, and large stretches of Game 3 before Smart had that insane game.