Cheapest onsite option would be the First Cabin in terminal 1 (capsule hostel with all the standard drawbacks) which usually runs about US$50.

Otherwise the Royal Park seems to be the cheapest onsite option with a private bathroom and actual soundproofing from other guests.

If you want both a low price and a private bathroom then this would require accepting the need for a short train ride and book an offsite business hotel in kamata/kawasaki etc