In addition to the fact that Sale arguably has been the best pitcher in the AL last 5 years and is only 27, his contract is very team friendly. Sale is under contract for the next 3 years (including team options) for less than $13 million a year on average (by way of example, the D'Backs will pay Zach Greinke $100 million+ over the next 3 years; if Sale was on the open market, he would get more than Greinke). So, a team can acquire Sale and it won't keep them from signing other studs. Sale is the kind of player that immediately would make the Red Sox, Cubs or Nats the favorite for best record in baseball. As promising as prospects are, no prospect in the game carries the same likelihood of success.
For the White Sox to deal him, their trading partner is going to have to offer value to the point where it hurts.