You can get from New York to Washington DC by bus for $20 one way or $35 round-trip. There are two bus companies, Apex http://www.apexbus.com/ and New Century http://www.2000coach.com/. They have slightly different schedules, but each charges about the same amount, and offers a similar experience.
In New York, there are two locations where the buses picks you up in mid-town Manhattan, and Chinatown. Apex picks you up at Penn Station, in front Madison Square Garden, and New Century picks you up at Port Authory. They both will also pick you up on Allen St. They both will drop you off in Chinatown in Washington DC, near the Verizon Arena.
Apex will pick you up either in two New York locations, Chinatown (28 Allen St., New York, NY) or Penn Station, and drops you off at 715 H Street. New Century picks you up in Chinatown (86 Allen St., New York, NY) and drops you off on 513 H Street. (You should note that both companies have their Chinatown location nearby each other. Don’t go to the wrong Allen St., location in Chinatown. The man behind the counter will look at you with complete disdain.)
On the return trip, there is a Starbucks, so you can wait there. Full Kee for some quick Chinese food. There is also a McDonald’s, and a number of bars and eating establishments in Chinatown.
Either company’s bus ride is a relatively pleasant trip. However, bring your own snacks and drink because there is nothing to purchase on board. Both buses will stop once for 10 minutes at a rest stop, a bathroom break for the bus driver, and you can pick up some snacks. Sometimes I am able to get both seats, and sometimes I am not. Just kind of depends on the time of day. Apex even has wifi onboard. (If you need to stop in Philadelphia or Baltimore, double check the websites.)
Comparatively, the cost to take Amtrak from New York to Washington, even departing at 3AM in the morning is $49 each way, more during peak hours, $74-135. Of course, flying is more expensive, and you have to take the bus or train out to LaGuardia Airport, Kennedy Airport, or Newark Airport. After you get to the airport, go through security, and wait 30 minutes before takeout that is two hours of your time. Then, you have to get in from Washington National Airport on the Metro into the City.
All in all, it takes the bus might be slightly more time, but its convenient, less expensive, and they are always happy to have a last minute customer.
Save even more if you are flying out of Dulles, up to $75 round-trip. http://bit.ly/8XEMeR