I'm pretty sure that, when the time comes, Sanchez will be released, not traded. That would put his trade value at zero. Whether he eventually turns into a capable backup, or even starter, years down the line, who knows.
But seriously, some of the posters saying Sanchez is worth something have me wondering. Would you actually prefer Sanchez starting on the Dolphins over, say, David Garrard? I don't get it. Why on earth would you?