It's been brought up to death. I've brought it up only 15 times myself.
He's a $12M cap hit to trade, but it's a "net" cap hit of $3M. $3M is locked into hitting 2013's cap no matter what. So you don't count that twice. Plus his $6M salary for the year.
Lose him next year and it's a pure $9M cap hit, which is a pure net $9M hit.
Keep him via an extension/reworking of his current deal and add $3M/year to whatever numbers we agree upon for the cap hits through 2016 for money we've already paid him in the past (your $12M divided over 4 years). So new deal worth $17M/year is effectively $20M/year, etc.
He isn't worth that, and the reason isn't because he isn't a great enough CB.
We'll see what his real demands are. No one wants to trade Revis just for the sake of trading him. He is not affordable long-term (even less so for us than for another team because of his prior bonus money I mentioned), and a long-term deal doesn't mean squat to him. Barring a similar clause Tannenbaum put in there, where he get extended forever at cheap dollars if he holds out for even 1 hour, he's just going to hold out again in a couple of years. And I don't see him signing another contract with that language in it because he clearly wanted to hold out yet again before the 2012 season.
We can't afford him long term, can't trust him to not hold out again if we can find common ground on a reasonable deal, and are going nowhere this year with or without him. Get something for him while we can unless he agrees to a $10-12M/year deal with aggressive anti-holdout language in it (meaning salaries are guaranteed rather than a lot of signing bonus money being paid, and those guarantees disappear if he holds out and his non-guaranteed salary drops to $3M/year). He'll never sign that so get something for him while the iron's hot.
Sperm our capologist supreme! WE or any team can not handicap ourselves tying too much into one player- so if we can not make it work we have to explore trading Revis