I'm just saying for this to "work out" he really needs to be a 4+ win type player. Like a Pedroia or Utley or Cano type 2B. Even a Howie Kendrick will be extremely disappointing compared to the amount of international talent you could have signed for $60 million.
If he's a 4+ win 2B, I think you're right - they probably won't care how much they spent on him.
What I've seen is that he would be the #10-15 prospect in MLB, and players in that range average 10 -15 WAR in the cost controlled years. I don't know the break even.
One thing to remember is that, due to prior signings, the Sox were going to be out of the international pool for more than $300K signings for the next two periods. This was sort of a use it or lose it opportunity to bring in a very high level talent. This does not impact their ability to sign the #7 pick in the Rule 4 draft this June, and it has no lasting MLB salary cap implications.
For the Nats, I don't think they are in the position of being limited in the international market. They will get a chance to play next year without having to compete with the NYY, BOS, or the LAD. That may be strategery right there. Of course, if MLB now changes the rules on the international FA signing, well, the stategery does not play out.
Another comp folks have suggested is the signing of Soler. Soler was not signed with the international pool limits, but he was a guy who needed development and went for $30MM or so.