There are a couple of discount buses that make regular runs to DC from NYC. Megabus and Bolt Bus are the most popular; I think Megabus tickets run about $20 but you'd have to check their website. One note on those: the bus stations in DC are in what we locals like to call "transitional" neighborhoods. Megabus drops off on North Capitol street, so if you want public transit, you'll have to walk 5-6 blocks to Union Station or Mt. Vernon metro stations. There are always plenty of cabs. But be wary coming in late at night.

Amtrak now has $49 tickets. At that price, definitely the best option. You won't get stuck in traffic and it drops you off at Penn Station near midtown.

I bought tickets on http://gotobus.com cheap but may be not the more comfortable way to travel