Not all fares are low but many of the cites served by the major airlines are very competitive. This combined with the window of allowable time to book the flight mentioned above by B747-437B make for good fares. The key is to book fares from one competitive city to another. As an example, ORD-LAX is a good fare right now at under $200.
Its better to be on the ground wishing you were in the air than be in the air wishing you were on the ground. Fly safe!