Look at budget bus companies like Megabus, Boltbus, or Peter Pan Bus when seeking cheap bus fares. According to WanderU, traveling at off-peak times can lower your bus fare; WanderU.com is an integrated bus search engine that aggregates competitor's prices for you.
Planning in advance allows you to avoid more expensive travel days and times. Sunday afternoon and morning hour commutes, when people are returning to work, are always more expensive. Prices tend to come down by Wednesday and Thursday. One-way $1 tickets between East Coast cities are available through Megabus and Boltbus, but do not count on the fare to be the same for the return trip. The times for these trips are usually very early in the morning, around 6 a.m.
Budget Travel's site ranks budget bus companies. As of 2015, Megabus serves the greatest number of cities in the United States, with locations in the Midwest and Pacific Northwest. Boltbus serves the Northeast and the West Coast. Tripper Bus serves a limited number of locations in the Northeast.
Lux Bus America serves locations in California and Nevada. Vamoose has stops in New York, Maryland and Virginia. Red Coach serves Georgia and Florida, with locations in Gainesville, Miami, Tallahassee and Tampa, among others.