I would be curious to see the details. First off let me say this.
I HATE PAYING GUARDS BIG MONEY. I think one of the things which caused the Patriots issues for years was the Mankins deal. Yes he was a very good player. But to have the highest paid guy at one of the least impactful positions, to me, hurts your ability to win championships. They were unable to be really active in free agency for a number of years. It wasn't until 2014 (the year they planned to trade him away) that they felt comfortable making moves. I doubt they sign Browner/Revis if they don't have an idea in place about trading Mankins and dumping his salary.
That contract was signed in 2011 They paid him for 3 years of play for 5.5/7/10M in 2011, 2012, 2013 and traded him in 2014. In 2014 they had to pay him 4.25 in dead money and in 2015 they had to pay 4M in dead money. (That is an average of 7.5M for 3 years without counting the dead money and 10.5M for 3 years of play counting the dead money).
You can factor in that they got a 4th round pick as well for signing the deal in 2015 (as that is what they eventually traded him for).
This deal however isn't as bad as the Mankins deal relatively speaking.
in 2011, 2012, 2013 the cap was 120, 120.6, 123M. For the dead money years the cap was 133, and 143M.
Considering this contract won't start to take effect until 2019 when the cap will be over 180M this deal looks a lot better even if the cap increase slows down after 2019.
There's about a 50% increase in cap space compared to 2011.
That means in a player worth 6.67M in 2011 is now worth 10M in 2019. Maybe slightly more as the cap was stagnant for most of those 3 years and may go up even into 2020 to some amount.
Masons deal is clearly the better of the 2 even if he isn't as good a player as Mankins
All that being said. I still don't love paying a guard this kind of money but it isn't a deal that i think will handicap the Patriots ability to sign other good players in today's cap age.