Rather than getting into Austin in the late evening (which my original flight did), my Dallas flight landed at 1am. I had to be in Austin for work the next day, so I rented a car and drove the 3 hours to Austin, getting there at 4:30 am. The car rental was only $30 ish dollars, but I had to pay about $200 to drop it off at a different location.
Not sure if there were better deals. There weren't many car rental places open at 1:30 am when I got to the car rental place at DFW.