Austin–Bergstrom International Airport (AUS)
Austin really only has one option to get from the airport to downtown Austin: car. You can take a cab or an Uber or Lyft as they are legal again in Austin. Generally I opt for Uber when going from Austin Airport to the city. Typically it's about $20 to $25 depending on where in Austin you are staying and roughly a 20 minute drive. Beware that Austin traffic is notoriously bad so if you land during rush hour expect the ride to be fairly unpleasant.
$20-25 from the airport to downtown Austin
Generally the cheapest time to book flights for Austin City Limits will be about six weeks before Austin City Limits. This means you should be focused on booking your flight in Aug. 24, 2018. This may seem late to you planners out there, but statistically it's the cheapest time to book flights. That said, if this is too late for you book it earlier. One final note is that plane tickets for Austin City Limits tend to skyrocket about two weeks out, so certainly don't wait that long to book a Austin City Limits flight.