Cap wise Revis and Sanchez are not connected. Sanchez biggest cap hit is in 2013, Revis is signed through 2013. If we choose to extend Revis we can easily do so, as our cap is in a lot better shape come 2014.
Talent wise QB is clearly our biggest issue, but we also have issues at WR, TE, OL, RB, LB, S (and probably a few more) ... this is why we need to reload. Whether trading Revis is worth it really depends on what we can translate his value into - trading him will itself create an issue at CB (unless you want to have Kyle Wilson starting all 16 games).
If we somehow parlay his trade into finding a good QB, then yes it will be worth it for that, but what if we turn his trade into a Guard and a Safety, for example? I don't see the value in that.
Ultimately what trading Revis comes down to is whether or not the team want to invest top dollar at the CB position moving forward. If the team feels that no CB is worth more than $12m, and Revis wants $16m, then it's time to move on, get what you can. If they feel he could be worth up to $14m, then there's possibly room for negotiation.
One final thing that occurred to me just the other day, when some posters were talking about Revis' value to other teams ... what if Rex gets fired next year? We could end up seeing a shift in defense towards zone coverage rather than man - how much is Revis worth in that case?