VivaAerobus is offering $49 fares, including all taxes, from Austin to all destinations which, in case you're not familiar, include Cancun, Monterrey, and Puerto Vallarta. Yes, cheap! But travel lightly, as these fares are for those folks not checking bags. Carryons only, people.
This sale is good through January 15, and good for travel from January 19 through June 30, with blackout dates of January 30 - February 2, March 13 - 16, April 3 - 19, April 30- May 5, and May 14 - 17. When booking, look for the -$.01 fares (yes, that's a negative), which is actually their VivaLight (aka traveling with a carryon only) discount, subtracted from a $9.99 base fare. Got that? Good. Add in taxes and, there you have it. $49 to Mexico.
Return fares at -$.01 are available too, just add in an extra $10 to those taxes, for about $59. The grand total? About $108 round-trip, taxes and all.
Folks in Houston and San Antonio can catch the VivaAerobus shuttle (as in bus) for $20.