You're worth what the market is willing to pay you. In relativity to what others are making (and soon to be making) at his position and interior DL he is absolutely worth the money he is being offered. Now, I also believe the market for DL is ridiculous and don't believe anyone other than a QB should get anywhere near $20 mil a year. But, it is what it is and players of Miller's caliber on the DL won't be had for anything less than $17/18 mil a year from here on out.
I also should add, that at this time Denver has no real money tied to QB. Now is a time they can afford to put big money into their defense. And you don't just let guys that can single handedly wreck a game (IE: SB 50) go in the prime of their careers. So, they kind of have to pay him whatever they have to pay him.