I think there's no doubt it cost him money, the debate is how much. For the most part I'm guessing they had formed their opinions on him so I think his value fell some but maybe not as much as some others are speculating. But performing poorly in those games showed his play got worse when it counted most. If he had come out and played lights out and led his teams to win and maybe got the Vikes to the Super Bowl his value would have increased some but still think ceiling was capped.
I don't think that's any different than people thought going into the season though. In fact, it's significantly greater than was expected of him going into the season. The expectation was that he'd lose his job by week 7, after the BYE. This was widely discussed, and emphasized after JJM looked good in the preseason. If you told Vikings fans that they'd be a playoff team & win 14 games, they would have laughed in your face.
So the reality is that no one was going to sign Darnold to a "top QB deal" regardless of whether he flamed out in round 1, or in the NFC-D, NFC-C, or in the SB. He led a team to 14 wins, and put up convincing numbers doing it. That's what will set his market value.
He had a good year. That raised his floor. He's going to get a 3 year deal, or get franchised/traded then sign a 3-year deal. That's why I don't believe it cost him a substantial amount of money. Maybe he would have gotten more guaranteed. Maybe not. It was 1 good season. He showed he can QB a good offense. I don't believe any team or scout was waiting for his post-season performance to decide whether he'd become a hot commodity on the open market. I seriously doubt anyone was going to give him a 5-year deal, even if he went to the SB, which I don't believe anyone actually thought he had a chance to do. I certainly didn't have him getting past the Lions. I thought he had a good chance against the Rams, but they showed up incredibly prepared. They went to school with DET's last game & had a week off to rest up for it.
I agree with Greg Cosell - there are maybe 3 or 4 QB in the league who could have overcome what the Rams threw at him. The Vikes OL was no match for it, and the Vikes OC certainly didn't make adjustments. He was battered and rattled - he certainly didn't play his best football. Absolutely no argument there. But people seem to love bashing Darnold, instead of crediting the Rams. It's a team sport. Between coverage and a relentless blitz, they owned the Vikings offense.
But he is arguably the top FA QB in a year with a bad QB class. He's going to get paid. That's really the long and short of it. I have no idea how anyone could know if he actually lost money or how much from those last 2 games. I know it's a popular narrative here, but respectfully, I'll take Cosell's word over FBG sharks. He's certainly studied more than we have, seeing as that's his job and he's been doing it for a long time. He specifically said that there was a lot more to see on the all-22 than one could divine from watching the game on television, and he didn't believe Darnold cost himself a penny based on market conditions & his regular season performance.
The article posted above seems to agree that it's a seller's market, so regardless of Darnold's last 2 games, he will be the one setting the price. All that matters is that a team is willing to meet it. And I'd about guarantee someone will. And if it's more than 1, he could even get more than expected.
The fact is that despite his poor play the last 2 weeks, he wasn't carried to 4300/35/12 - he achieved that. He had a 102.5 QBR, which includes that last game against the Lions. Sure he had good WRs and a good coach, but that's the body of work. Whether you or I believe it's replicable isn't really relevant. All that matter is that an NFL franchise believes it.