Cheap flights to Washington, DC range from $191 to to $445 (inclusive of fees) if you're flying from, say, Houston, Texas. If you're coming from a farther city, Los Angeles for example, Northwest Airlines offers the cheapest price at $183 with a 5-day advance purchase policy. Other airlines such as American Airlines and AirTran Airways charge $285 for a roundtrip economy class ticket from Los Angeles to Washington.
The good thing about flying to Washington is that it doesn't seem to matter much on what month you fly, prices don't really fluctuate that much between the months of September and December. AirTran Airways and Virgin America, for example, offer rates between $285 and $311 with December 2008 as the latest date (still from Los Angeles). All in all, Northwest Airlines seems to offer the cheapest airline tickets to Washington no matter what state you're coming from.
Below is a list of Washington airports that you might find useful:
Flights
