There are three hotels within walking distance, but that also offer van service to the airport, the Sim building and HQ.
Holiday Inn - Dulles Airport (Route 28)
Hampton Inn - Dulles (Holiday Park Rd)
Towne Place Suites by Marriott - Dulles (Holiday Park Rd)
There are no discounts other than something you can work out (like AAA membership, AOPA, etc). They are usually quite expensive unless they are expecting a low occupancy night.
Holiday vans are White, Hampton are Silver and they run on a schedule that depart the hotels on the hour and on the half hour. They are at the airport usually quarter past and quarter to the hour. When you walk out the main terminal on the baggage claim level turn and walk to the extreme Eastern or Western end of the bus island (I think the station numbers are 2A and 2G). You can take either van to any of the three hotels.