The $399.99 on lifetime is the best you can get. This is the MSD price because its a secondary service box on your account. Should the first box fail, or not even connect to Tivo regularly, the next Tivo you add will be $499.99 on lifetime.
Original S1 (Recycle Heaven)
Series 2 (PLS, sold)
TiVo HD (XL, PLS, "652")
Juliet Basic (4XL, PLS)
The TiVo Apocalypse begins on Sat 2013 Feb 16 00:00 GMT!