The Shinagawa Prince Hotel is right outside Shinagawa Station, which is convenient for both the bullet train and the ordinary train loop. And because Shinagawa is at the bottom center of the loop, it's easy to get to both the east and west sides of Tokyo.
And there's a lot of convenience in the hotel, too. "The complex features 15 restaurants, a range of cinemas, an IMAX
theater; the sporting facilities include indoor pool, tennis court,
indoor golf range, and even an 8-lane bowling alley."
You can reserve a room at the Shinagawa Prince for $79 per person per night. It doesn't get any cheaper as you add more people, but it's still a reasonable price for the neighborhood and the convenience.
Check out the hotel and make reservations at NTA.