It appears that Telcel provides a Pay-as-you-go internet service. I have not spent sufficient time trying to translate all of the information available on their website to see how one goes about doing it, but after 10 minutes or poking around, it looks like it's pretty straightforward. The next time you are in town, you may want to stop by the Telcel main office at the Centro Maya shopping mall and speak with someone about the setup process and how you can re-activate your USB after you are out of Mexico for a while.
Here are the pages that provide you with some information, in Spanish. If you can't read it all, at least you can get an idea of what types of equipment is available and the monthly plans available.