The way I see it.
1) The Twins almost have to deal Santana for many reasons. a) It is unrealistic for them to make the playoffs with Detroit vastly improved and the Guardians in their division. If they let Santana walk, they get 2 1st round picks, but they most likely won't end up as good as the packages any team is offering. c) They have many holes to fill and the Sox prospects are good and inexpensive.
2) The Twins don't have much leverage since no team is "really into" Santana as much as the Sox, and they have the best offer now. Since they are trying to squeeze Ellsbury and Lester out of the Sox, they act like they won't deal him. The Sox know this so they are standing pat. Which is why it wouldn't surprise me to see this deal happen next week when there aren't 1000 reporters annoying them.
3) The Sox have a good package. Look, for most all-star pitchers, it is a top prospect and then a mid level one. Ramirez and Sanchez for Beckett. They also got Mota and Lowell which is why they gave up more. Pavano and Armas Jr. for Martinez. Also, these guys were under contract for awhile, unlike Santana, who you have to fork over 100+ million. Lester=Top lefty pitching prospect who could be a potential #2, Masterson=Potential #3-4 Derek Lowe type. Coco is a gold glove caliber CF, who could turn his offense around in Minny. And Kalish is unknown at this point.
4) For the Sox side, I am sorry, I'd deal Ellsbury AND Lester for Santana. As fun as Ellsbury is to watch, he isn't going to win a WS title unlike Santana could. We are over obsessed with him. What if he hits .258 next season and Lester goes Craig Hansen on us and posts a 4.9 ERA while Santana wins a Cy Young and goes 20-7 with a 2.22 ERA?