If the Sox don't re-sign Nap it's not inconceivable that the Yankees could be interested in him as DH/backup 1B.
Nap's career line vs. NYY 342/457/651
Nap's career line vs. Bos 288/379/696
Signing Nap away from from Sox would be an excellent, spiteful PR move, pissing off their hated rivals, getting a player who murders them and also has great numbers vs. us.
As for his cost, well, they did pay Youkilis $12 million to be a stopgap for A-Rod.