Unfortunately, the conference hotel, Rodeway Inn-University, is now full.
The next closest hotel, and the only other one within easy walking distance of the Mathematics Dept, is the Days Inn at 3105 N Interstate 35 (512-478-1631 or 800-725-ROOM). It is across I-35 from the Rodeway-University and has been remodelled recently. The UT weekend rates are: Single $72, Double $79, King Suite $85 (has king bed and double-size sleeper sofa--good for groups).
If you will be driving and price is an issue, I suggest that you try one of the Motel 6's on I-35--either the Highland Mall or the North location. Be sure to ask for UT rates.
In addition, The Rodeway Inn-North is actually quite close, 5656 N. Interstate 35. Its weekend rates are: Single $48, Double $56.
If you are driving and price is not an issue, almost every major chain has a hotel in central Austin.
The hotel tax rate in Austin is 13.5%.