My guess is that the Nats are willing to go five years, maybe six, @ around $22M, with an opt out (player opt out) after three. And, that there are no teams willing to go higher.
Texas may be the only team that would go higher, in theory, but if Texas signs Darvish they are out of the picture and I fully expect that they will. The speculation about Cubs interest is ridiculous.
If I had to bet right now, I'd say we sign him, but not until Texas signs Darvish.