I always found this to be the most interesting part of the MicroCell fine print. WTF is wrong with AT&T charging me twice for the same data, really.
The Microcell use does not incur any additional charges, when you use your phone with it uses the minutes that are part of you wireless plan. That is the regular rate they refer to. You have the option of paying $20 a month to have unlimited minutes during Microcell usage however, no matter what the maximum contract minutes are for your phone.