You would be paying that price, if BB were as dumb as the Draftnik posters here, who would trade Mallett for another draft pick.
They NEVER include replacing him with another body. It seem the going price for a realistic, backup QB is around $4-5 million a year now, judging by the Cassell and Fitzpatrick deals.
It is also the likely lowest transition tag level that Mallett will command in 2015. Thus a subsequent 3rd round compensation pick when lost, or Pats price if kept after next season.
BB gets to keep $4 million in CAP for 2014 paying Mallett under a
$ million. Essentially the same CAP "savings" that a renegotiated Wilfork deal might achieve, with no fuss.
I think guys like Fitzpatrick, Vick, Cassel, Schaub, etc aren't good examples of the typical "backup QB" position--by any stretch.
They are all basically journeyman vets who have the capacity to start in a pinch at any given time, due to the uncertainty of their teams QB1 situations. They are also falling in the range of 2.5 to 5 million dollars per year, with Cassel at the highest end--since he's almost guaranteed to be starting at least some of the games, if not
most in 2014 over Ponder.
Even in a worst case scenario, they may offer QB1(b) status, and "push" the guy in front of them like Geno Smith, Christian Ponder, etc to either do better, or ride the bench for a handful of games.
If we're looking at the truer definition of backup QB positions, we're looking at salaries much lower than these examples, and I'm not as sure as some that I'd include Ryan Mallett in this grouping as of yet.
To me, Mallett is a really odd kind of case in some ways when trying to predict what could happen to him, at least aside from the fact that he'll almost surely go somewhere else where he'll have better odds/opportunities to compete for more playing time. Other than that, we really have no idea if he's a QB1 or a QB3-4. It's very much up in the air.
For what it's worth, I do agree with your thought that it's better to keep him for the 2014 season at the price he commands; but I highly doubt Belichick would be replacing him with anything of a backup who'd be much more expensive.