There's low fares in them there Codeshares

Q. I recently purchased a roundtrip ticket from MEX to ORD for my in-laws. The ticket was purchased as a codeshare between USAir and United (USAir was the ticketing airline and United was the operating carrier). The flight purchased through USAir was roughly $200 cheaper than the same exact flight would have been had I purchased it through United. Why is this? Is this typical?

A. This often happens, especially on international flights. Whenever you see a codeshare flight, it's a great idea to check both airlines to see if one has a lower fare. Airlines buy blocks of seats on competitor flights, and if they can't sell as many as they thought, they'll discount them rather than see them go empty.