If you're looking to make your MR into a matress run too and want to do this on a budget, the only cheapo option I'm aware of is the Holiday Inn Tobu Narita.

It's not great, but the rooms are better (and bigger) than the standard rooms at Le Meridien Pacific (near Shinagawa station - which gets a big thumb down from me). Courtesy bus from the Holiday Inn to the airport is free and runs every 30 minutes or so, you can also get a bus from here into Narita Town.

Rates run from about 6,000yen per night per room.

I've done everything possible to maximise my hotel experience and comfort with my own limited 'own dime' resources in Tokyo over the past 4 years. It's very rare that inventory just 'opens up' a few days before your stay. The best bargains I've found have been booked well in advance.
(my favourite has been using Utell.com to get cheap rates at the Takanawa Prince)