I agree that Dulles is too far out (and "Dulles" encompases a huge area) BUT, if you have a car and can drive to a metro station, I wouldnt totally rule out the reston/herndon area. The reaosn I say this is because Vienna Metro station is very accesible from these areas and hotels there can be much less expensive.
The Residence Inn Fairlakes is about 6 miles from Vienna Metro station. 2 bedroom suites are about $130/night. so $65/night for each of you. There are also a lot of restaurants near this hotel so if you are back at the hotel for dinner any night, you wont be searching for places.
The Springhill Suites in Fairfax could also be an option. Its about 4-5 miles from the Metro station.
Personally, we lived near Dulles airport for almost 17 years. We lived near the fairfax county/loudon county border. We never found it to be a huge hassle to use the Vienna Metro station. If cost is really an issue adn you want them to go with you, I would not rule out staying near Vienna metro.