I decided to actually do some research and quote each airline's CASM from their respective 10-Q reports:

For the three months ending 30-Sept-2010, SkyWest INC CASM: 8.9 c/mile
For the three months ending 30-Sept-2010, XJT CASM: 7.02 cents/mile

Notes:

- Both operational expense rates exclude fuel.

- SkyWest INC does not publish separate CASM reports for both operations in the 10-Q; however, it is safe to assume that ASA is more expensive than the averaged SkyWest INC cost. The last publish CASM specific to ASA that I remember was 9.2 cents/mile, and given the published data, its likely a bit higher than that.

- Using 9.2 cents/mile, XJT is 23.7% cheaper than ASA