VivaAerobus, a low cost Mexican carrier, has a sale with base fares of -$0.01 (yes, that's negative one cent) or $9.99 each way on its non-stop service between AUS and CUN. The former is for a "vivalight" fare, which allows only carry-on luggage, while the latter allows a checked bag plus a carry-on item. Considering that the taxes to Mexico alone run around $100, this is about as good as it gets. Other Mexican destinations served by VivaAerobus are also on sale. You must purchase by December 10 and fly between January 7 and February 28, 2009. I took a trip with my family from AUS-CUN on VivaAerobus in July and the service was no frills, but otherwise perfectly fine. Book here.