I am writing this from deep in the heart of Red Sox Nation, where I am visiting family. As you might expect, everyone here is a hick, and this is being written on the only computer in a five-mile radius. All I have been hearing is, "Don't worry, maybe the Bruins won't win the Stanley Cup," as well as the proclamation of Jacoby Ellsbury as the second coming of Jesus and how laptop-boy is better than Hughes.
Anyway, I found this article by Vince Gennaro, a baseball economist. He analyzes the possible Johan Santana trades and it is very interesting, you should definitely check it out. With regards to the possible Santana to Yankees deal, he says it does not make financial sense for the Yanks. He describes the prospects necessary to complete the deal as a tariff, because Santana would require a large sum of money to be signed long-term. I agree with this 100%. He doesn't say it specifically, but implies that it would make sense to sign him as a free agent after this year.
He later adds that it makes even less sense for the Red Sox, but a case could be made for a Mets trade.