There is a super cheap way to travel across the United States using intercity buses. Back in the Summer 2013 I made a long journey from New York to Houston, Texas for about USD 200 per 2 persons using MegaBus.
Last time I was in the US, we decided to go to Philadelphia from NYC using Greyhound, the ticket price was very cheap some 24 USD, again for two persons. When we traveled back to NYC, we bought even cheaper - USD 10 tickets for return way.
Either you are on the budget or you just love to travel by buses, greyhound and megabus are great choice.
The Philadelphia Greyhound Terminal is the primary intercity bus station in Philadelphia, Pennsylvania. It is located at 1001 Filbert Street in Center City Philadelphia, immediately north of The Gallery at Market East shopping mall and the SEPTA Jefferson Station, a few hundred feet east of the Pennsylvania Convention Center and Reading Terminal Market, a few hundred feet south of the Trocadero Theatre and the south edge of Philadelphia's Chinatown district, and within 1/2 mile of Independence Mall, the U.S. Mint, and City Hall.
Greyhound bus station in Philadelphia