I think the combination of SJC, AUS, and SDF usually signals flying American Airlines, at least for the connecitons. Generally, most airlines will be matched by at least one other airline in a market (say, SJC-SDF will usually be priced the same by two or more airlines). That helps to keep the costs down. I would start researching historical fares on the routes you want to fly using farecompare.com and farecast.com to get an idea of what you might expect to pay.