You don't mention the day of the week; Dulles area hotels can be more expensive mid-week ($100-150), but are often dirt-cheap ($60-80) on Friday and Saturday nights.

If you do rent a car, Dulles returns aren't bad, just a 5 minute shuttle ride from the lot to the terminal. And there's a reasonably priced gas station on the airport grounds, right around the corner from the rental lots. If you don't want to deal with the bags and family on the shuttle, just drop the wife and toddler and bags at the terminal, then go by yourself to return the car.