Alaska Airlines and Southwest airlines are the two airlines that have the cheapest unaccompanied minor fee for flying alone.
Alaska Airlines charges a $25.00 unaccompanied minor fee for nonstop or direct flights each way and Southwest airlines charges a $50.00 unaccompanied minor fee each way.
Other airlines such as Spirit, JetBlue and Frontier charge a $100.00 unaccompanied minor fee each way and the airlines US Airways, United, Delta and American Airlines charge $150.00 Unaccompanied minor fee each way.