Would the Patriots really want to open up too much cap space with the speculated long-term Mankins deal?
Right now he has a cap hit of $10 million. The Pats are slightly under the cap using TOP-51 accounting, and probably real close to the cap with Top-53 + practice squad if the season started tonight. The Pats will most likely drop a couple of million bucks in cap obligations with the roster cut-downs to get to 53.
If the Patriots are relatively satisfied with their roster as it is constituted on the day that Mankins signs his contract, then the incentive would probably be to keep future years as cap-flexible as possible by minimizing the signing bonus.
For instance, a 6 year contract with a $15 million dollar signing bonus, a $6.5 million roster bonus and a $1 million dollar salary in 2011 has a 10 million dollar cap charge while it delivers $22.5 million into Mankin's bank-account. Doing the deal that way only leaves a $2.5 million per year pro-rated signing bonus while doing a $21.5 million dollar signing bonus and a million dollar base salary for 2011 gives Mankins a cap hit of $4.7 million this year, but annual $3.7 million dollar prorations.
Assuming Mankins continues to play the way that he has played, worrying about pro-rations of the signing bonus is a non-issue for at least another three years (2014 season), but this strategy of a decent sized roster bonus and no significant reduction in his cap hit buys future flexibility. And if Mankins gets hit by a bus, the cap cost of dealing with that scenario is les even though the cash cost is the same.
