How Much Will AT&T Charge for iPhone WiFi Hotspot When/If it is Offered?

The Verizon iPhone 4 has one significant feature advantage over the AT&T iPhone 4: Personal Hotspot WiFi sharing. This feature lets you share the Verizon iPhone 4’s 3G wireless data with up to five WiFi devices (e.g., WiFi-only iPad, notebook, mobile game consoles).

AppleInsider reported that a developer claims that AT&T turned on the WiFi hotspot feature for his iOS 4.3 Beta 3 tests.

AT&T reportedly provisions iOS 4.3 beta Personal Hotspot feature

It seems reasonable to expect AT&T to make this WiFi hotspot available for their iPhone 4 users in the near future to compete effectively with Verizon. The questions are: How much will AT&T charge for WiFi hotspot and how much data per month will the option include?