From Indy you can get non-stop flights on SWA, ATA, NWA, and Airtran. Neither Southwest or Airtran have there October schedule out yet, so NWA and ATA have no insentive to discount.
Southwest will probably have the lowest prices - generally you can find $59 fares - that comes to $137 rt after all the taxes. You can usually get those rates if you are flying mid-week. If you are flying weekend you may need to watch Southwest website and grab the flight soon (like in the first few hours) after the October schedule is announced to get those rates. Airtran is currently $74 - approx $170 rt for the August timeframe - expect the same rates for October. Many times Northwest will match Southwests' rates, I do not know about Airtran since they are new to Indy as of April 9.
Booking a October trip with ATA from Indy is a very risky bet. As of April ATA will have only 4 flights from Indy. In my opinion 4 daily flights is not sustainable, I would guess sometime over the summer they either increase to about 10 flights or they leave Indy altogether. Long term, no one knows what will happen with them.
A couple other things you may wish to consider.
You will probably get your lowest fare from a discount carrier. I believe most of the discount carriers in Chicago fly out of Midway not Ohare. I don't know if that matters to you - my parents live NW (between Chicago and Rockford), they will always take the 45 minute drive to Ohare over the 90 minute trip to Midway unless the flight is much, much more expensive out of Ohare.
Fall Break has become a big deal here in Indy (as it has many other cities). Generally Fall Break is only 2 or 3 days, but many parents are taking kids out of school for the whole week. Some schools have Fall Break the 3rd week in Oct, others have it the 4th week. This will affect airplane seat availablity.
Have a good trip.