I'd place the probability of an extension as somewhat higher than .5, given that 1) Revis did leave money at the table last year, when he could have gone to a non-contending team for a higher payday, 2) familiarity and presumed comfort with the incumbent team, 3) legacy, and 4) Kraft's desire to get this done.
I suspect that Kraft will be more involved in the selling/negotiations process than for other players, given his comment to Revis in the locker room post-game (in the video posted in the pics/vids thread). Not that he would step over Belichick's final say, but I'm guessing he has been/will be stroking Revis' ego throughout the post- Super Bowl period. These things might tip Revis' consideration of the marginal value of staying with the Patriots over say, the marginal value of an extra 2 million/year from a non-contending team, when the Pats are offering $16 million/year.
I mean, who knows really, but think this isn't like the Lester situation, where it was pretty clear the Red Sox had no interest in being anywhere near market value.