At the minimum, Dak will make $28M per season. He's done more than Matthew Stafford, Jimmy Garoppolo, and Kirk Cousins, and deserves to be better paid than any of those players. Let's say, 6 years, 171M - an average of $28.5M, with a $45M signing bonus, and $90M guaranteed over the first 3 seasons.
2019 - $45M signing bonus, $3M salary = $12M Salary CAP hit - $90M Dead Money
2020 - $20M Restructure Bonus, $1M salary = $14M Salary CAP hit - $78M Dead Money
2021 - $21M Salary = $34M Salary CAP hit - $64M Dead Money
2022 - $23M Salary = $36M Salary CAP hit - $30M Dead Money - 4th year out, save $6M against CAP
2023 - $23M Salary = $36M Salary CAP hit - $17M Dead Money - 5th year out, save $19M against CAP
2024 - $32M Salary = $36M Salary CAP hit - $4M Dead Money - 6th year out, save $32M against CAP
DAK gets paid $69M over first two seasons - an average of $34.5M per season. He is guaranteed $90M over first 3 seasons, an average of $30M per season. The Cowboys defer the CAP hits on Dak for years 1 & 2 of the contract, but can decide to eat the 3rd year, and after that, they can actually save money against the CAP by cutting Dak.
Even at a CAP hit of $36M per season, the Salary CAP is expected to be around $220M in 2022, so that is only 16% of the overall Salary CAP - which is about right for a top QB. If Dak fails to continue to improve, then the Cowboys have an out in Year 4. But if he wins a Super Bowl or two over the next 6 years, while the top QB contracts rise to about $45M per season, the Cowboys will have made a bargain deal.