Before Allen signed his deal people here were saying he was only worth ~$16 mil/year and some even thought that was too high.
Sexton is a proven commodity who can give you an average of 20 points per game. Those don't grow on trees.
I'd say the Cavs should just revisit the contract they already offered and apparently Sexton turned down, whatever that amount was. The Cavs already calculated his value to them and made the offer. Sexton will not be worth any less than that amount. He might accept it now after the injury, or he might prefer to test the market.