Dak is easily better (and younger) than Foles. So, yes, he's worth more than what the open market says Foles is worth. Of course, he's not on the open market yet (and almost surely won't be until he's entering his age-29 season, at the earliest), so there's some wiggle room on all of this.
Of course, the cap rises and new contracts get bigger and bigger, establishing a higher bar for what a starting QB is worth, in the next 3 years... so Dak has leverage too because his extension gets more and more expensive the longer we wait (barring a career-derailing scenario like Robert Griffin experienced). Lots of premium position (like QB) contracts start out as "oh my god, this is a world-ending contract" and then a few years down the road, people are like: "Ha, look at teams paying even more for worse guys now; that team is so glad they locked him up sooner rather than later."
If we went to Dak and said, "we'll tear up your current deal and sign you to a 4 year, 88 million dollar deal," maybe he takes it just because he likes the security of having that deal in hand and he doesn't want to wait until the franchise tags kick in to start making big time money. (Though I suspect it would be more like Foles deal *plus* some very rich incentives based on personal or team achievements, allowing his agent to crow about the puffed-up version of the deal which may or may not ever be realized.)
Maybe he decides he'd rather lock in a new deal now before the Joneses dampen his value with any dumb personnel decisions or crippling coaching hires. The hilariously bad o-line depth and disinterest in giving him weapons to work with at WR and TE already temporarily dampened his value artificially. He might want to sign up for an immediate payday before the Jones brain-trust screws him again.
Or maybe his agents tell him, "Sign the extension and avoid the contract bickering. It'll just make you less liked, and we fully plan to ***** you out for as much endorsement income as possible, which depends on you being as well-liked as humanly possible. Plus, a prominent, likable former Cowboy gets to walk into a lucrative second career as a broadcaster. So let's stay likable and make sure you're raking in easy money for decades even after you quit playing."
Of course, tearing up his current deal and replacing it with Foles' 4-year deal, means the earliest he'd be able to hit the open market is entering his age-32 season (following 4 years under contract plus a couple of franchise tag seasons). Maybe that's a deal-breaker to him and his agent.
Or maybe Dak actually sees the upside of that because it means he can hit the open market while he's still got some gas left in his tank, either to strike it rich with one last big contract, or maybe he wants the option to hand-pick a new team to go chase a title in the twilight of his career if he's still ringless after suffering through the Jerry Jones Festival of Incompetence for the first 8-9 years of his career.