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.
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.