First of all, if this is in the wrong forum, please move it. I'm pretty sure this is the right place for it, but you never know.

My question:
What is the cheapest way for me to go from Washington DC to New York and back in the same day?

I'll be in DC for a few days and I'd like to go and visit my friends in NYC. I need to go from suburban DC to manhattan. I only have a day so travel time is essential. At this point I'm thinking Amtrak 3 hours each way for about $140 each way would be my best bet because I don't deal with airport time and airport to manhattan center travel time and taxi $$$

Can anyone suggest anything better? I'd really aprpeciate it