I fly PHX-TYS regularly and have noticed that the fares on US are typically $100-200 higher than AA and CO. I've been paying it due to elite benefits, but if you see my other post on the regarding my upgrade success on US flights, these don't appear to be much more than 100% bonus miles. I'm beginning to wonder how much longer they can keep up this pricing structure, but it seems like the planes are relatively full.