I guess with a P8 you would hope that the sensors( AN/APS-137B(V)5) are a whole lot better than the P3 so that you can cover a much larger area of ocean more quickly, although the electro-optical systems are probably not that much more advanced than the latest P3K update. The radar should be a reasonable step-up from the Israeli Elta system going in the P3K's.
I would imagine that 4 P8's will cost around the 1 Billion dollar mark depending on systems config, luckily the airframe is a known commodity and there are some economies of scale already there.
I would imagine that the the Orions will be used at least until 2020 with the last aircraft from the latest update not coming online until 2010, likewise with the C-130 program. Would'nt be surprised if 2025 was a more realistic replacement date, by which time the airframes will be 60 years old