I don't fly for work, so all my travel is personal and thus, costs are out of pocket. There are so many factors that I don't have a hard rule for flying vs driving (i.e., anything more than X hours, I'd fly).

Obviously, anything that would require me to spend the better part of a day on the road, I'd fly. But shorter distances, it all depends. If I'd have to rent a car at my destination, I'm more likely to drive. If hotel parking would cost more than a plane ticket, and there was decent public transportation, I'd be more likely to fly.

KC, Memphis, Indianapolis and Chicago are all roughly the same distance from my home here in St. Louis. KC, Memphis and Indianapolis, I'd definitely drive, because a car would be needed in those cities (besides I don't think there are nonstops from STL to MEM and IND unless I could stuff myself into a FedEx box). Chicago, well, then it all depends. Am I staying downtown where hotel parking is 50 bucks a night? I'd probably fly. But I'm going up there for a family function in a few months but it's in the northern suburbs where I'd need a car anyways, so I'm driving.