If $20 + mil is the market, then it's the market. As much as it makes me cringe. And you either pay it or not.
I hear people on this board make the claim that sure, the Rams can pay it and not have any effect on their ability to sign other players. Well, that simply defies the laws of math. There is a hard cap; there is only so much money to go around. Of course paying AD $15 or so mil more than he's making today is going to affect who else the Rams can keep or sign.

Is he maybe one of the 3 or 4 Defensive players in the NFL worth that kind of money? Yes. He has so much effect on a game.
But as Rams fans ought to have learned by now, an otherwise very good player won't stand out unless there are good complimentary players around him. If the Rams wind up with too many weaknesses on Defense because they don't have the money to fund the other positions with good enough players, the other team will exploit those weaknesses and Donald's heroics won't matter as much.

You have to figure out a way to have good solid players in every position if your goal is the Superbowl.

But I will say this: Donald is to DTs what Gronk in his best years was to TE's. There is no one comparable. The trick is to pay whatever you have to pay to keep your HOF type star players, but even then only to a point that other critical parts of your team aren't so compromised that the overall result is that the cost of keeping that star player means your team is actually diminished.

And of course the Rams know that the goal is to structure the deal in such a way as to allow them the most flexibility to keep or acquire those other complementary players. But if Donald's agents won't go along with a deal that does that? Then you use the NFL's rules to keep Donald as long as possible, including using the Tag for a couple of years. And as has been demonstrated, it's no big deal if Donald holds out and misses all of training camp and even Preseason to exhibit his unhappiness.