You never pay a receiver for what he's done. You pay him for what he will do. It took Jerry a LONG time to learn that. And if there are concerns over his back or behavior, that factors in greatly.
You never dump a huge percentage of your payroll on a receiver. This game is won at the quarterback position and on the offensive and defensive lines. Who are the great receivers for Seattle and New England? How many titles have Calvin Johnson, AJ Green, Julio Jones, Andre Johnson, etc, won?
As good as Dez is, he isn't especially fast, and he's limited in his ability to run route adjustments and whatnot. Pay him well, but pay him sensibly. Otherwise, franchise him and be done with it.