The only way Sanchez warrants trading a pick of any kind is if a GM thinks that he might flourish in another system and turn into a serious QB. I personally can't see that happening. Like Henne, Sanchez was not an instant bust, but it doesn't matter how skilled a failed QB is, he is simply a failed QB. One fatal flaw is all it takes. If a GM knows a QB can't take his team all the way, and the QB doesn't have some kind of athleticism to play multiple roles while also filling the backup QB role, then you don't give a 4th through 7th, you take him out of free agency.
Can you imagine the Dolphins trading a 5th for a QB that you know is not going to challenge for a starting role at this point?