If you do mileage runs, you probably know about matrix.itasoftware.com, but in case you don't - visit http://matrix.itasoftware.com/?showPricePerMile=true and then do your search to see and sort by price per mile.
What is a mileage run?
Avid frequent flyer miles collectors hunt for abnormally low cost fares which travel long distances (often due to multiple out-of-the-way stops). This results in a great cost per mile for them.