Need More Data? Answer Center Says You Can
Question:
What happens if I exceed my smartphone data plan’s monthly allowance?
Answer:
If you have DataPlus and use more than 200 MB, you’ll receive an additional 200 MB for $15. If you’re on the DataPro plan and need more than 2GB, you'll receive an additional 2 GB for $10. Overage is replenished as often as necessary during your billing period.
The Answer Center has conflicting information from their press release which says, "Should a customer exceed 2 GB during a billing cycle, they will receive an additional 1 GB of data for $10 for use in the cycle."
This would make pricing identical to the Verizon or Sprint MiFi which allows 5GB for $60. AT&T tethering plus an extra 2GB is $25+$20+$10 = $55 for 4 GB or $65 for 6 GB.
http://www.wireless.att.com/answer-center/main.jsp?t=solutionTab&ft=browseTab&ps=solutionPanels&locale=en_US&_dyncharset=UTF-8&solutionId=KB110272&isSrch=Yes