Is this for OCt 20-22?
What are your dates and what airlines are you willing to fly?
Houston will be a lot cheaper with San Antonio being a bit cheaper than Austin. Frontier has cheap flights to Austin and if you fly early enough before the event, you can get it as cheap as Houston.
Austin is 20 minutes from COTA.
San Antonio is 1 hour away from COTA and 1 hour and 20 minutes away from Austin.
Houston is 2 hours and 45 minutes away from COTA and 3 hours from Austin.
For cost, don't forget to factor in the cost of a car rental. They can vary by $100+
The closer to the event date you get the bigger the savings flying to Houston. If I'm saving more than $150, I would drive.
If San Antonio and Houston are close in price then I would go to San Antonio.
If you are flying several days before the event and can stick around for a couple of days after the event and you are ok with Frontier airlines, they have cheap tickets to Austin.