Just to put this into context I am an Eagles fan about to talk about a Cowboys player.
Dak is fairly good. If I was to compare him to a past player it would probably be McNabb. His stats will always look a bit better than they eye test, but overall he's a very good, young, durable QB who can absolutely lead a locker room.
Someone called him a 'diva'. I don't see it that way at all. This is Kirk Cousins version 2.0. Here's been his cap impact the last 4 years along with his 2020 number:
2016 - $545,848 (0.3% of cap)
2017 - $635,848 (0.4% of cap)
2018 - $725,848 (0.4% of cap)
2019 - $2,120,848 (1.1% of cap)
2020 - $31,409,000
He's been by far one of the best 'deals' in the NFL for the past 4 seasons. Now the Cowboys have rewarded him by giving contract extensions to Elliot, Smith and Cooper.
When I mentioned this was Cousins version 2.0 I wasn't lying. The Cowboys are using the franchise tag to create leverage. The problem with that is they are on the hook for ~$31MM guaranteed this year and another ~$38MM next year. Combine the two of them and that's ~$69MM guaranteed over the next two years.
Dak has next to no incentive to take a deal that has less guaranteed money than that. This is what happened to Cousins. When Cousins declined then Washington's GM, Bruce Allen's offer which was marketed as 'the most guaranteed money ever offered', he was including the franchise cap years pretending he didn't paint themselves into a corner.
Anyway, many have mentioned the main disagreement was 4 vs. 5 years. Cowboys wanted 5, but weren't willing to provide much more guarantee or inflated salary that last year. Dak wanted 4 because he can then hit free agency again when he's in his prime.
What do I think Dak will cost? He'll be more than Carson Wentz and Jared Goff. Those were ~$33MM/season with ~$108 total guarantee. With Dak's deal being 2-3 years behind theirs it will probably be ~$38-$40MM/season with ~$120MM in total guarantee.
One final item to bring up. The Cowboys are not in a great cap spot for the 2021 season. They only have an estimated ~$32MM in cap space with no QB contract on the books. If anything Zeke's agent should get a raise. Somehow they threatened a hold out and managed to leapfrog the QB and #1 WR for an extension. Amazing work his agent that ultimately forced the Cowboys into making some tough decisions.