Right around 15 seems like the right spot for Prescott. Actually somewhere between 12 and 18 on any given Sunday. His value is how he ranks relative to cap hit. Thinking about it off the top of my head I might put him top 3. I think Brees and Brady deliver for their huge hit. Rodgers I think is the best QB in the league but his cap hit and the Packers personnel ineptitude results in a 5 win team. Does that make sense?
Brees was a top QB 2014-2016, but his hit on the cap and the Saints personnel moves made three 7-9 teams so you could argue to value he was not a top 10 QB those years. Last year Case Keenum was a valuable player, this year he is not.
The Cowboys have about 45 million in cap space next year, and they've said they want to extend him early. If they do they're sure to pay him like a top 5 QB instead of a top 18 QB. The likely result is a 25 million + hit. That leaves little room to extend Elliot, and resign Lawrence. It seems you can do two of those things but not all three, which means you leave Elliot on a rookie contract, something very unpopular with star players, or you don't resign Lawrence. The other alternatives are letting go of one of those high priced OL contracts, or deciding an injured all the time Sean Lee isn't worth $10m.
In any event your team is getting worse around Prescott not better, and there's no room to sign any FA's outside the organization. In 2019 you're getting a 20-25 slot in the draft, and a division champ schedule, so things don't get easier. A harder schedule with a less talented team, doesn't make for a promising 2019.
The next year you're going to have to pay Elliot one way or another, and Cooper comes due. Huge cuts elsewhere will have to happen to keep those guys around.
That's the thing about QB's and the cap hit. If you're going to take a huge hit, they must deliver because other parts of the team are going to suffer. They have to be great to make up for what you're giving up.