We are equal distance from Indy, St. louis, and Louisville so I have been checking southwest and airtran from each since that's the only 2 I can find nonstop flights from. It looks like there's some decent SW fares this summer out of Indy but I don't know your exact dates.
I ended up booking SW thru st. Louis for $284/pp. When we flew 3 years ago we flew $100 cheaper pp out of Indy. Can't believe I just spent that much but was afraid they were just going to go higher!