The wap121, 321, 551 and 561 do support radius accounting. I don't know what price point you were looking for, but which model you choose depends on how many users and the capabilities you are looking for.
This doc describes how to configure it.
http://sbkb.cisco.com/CiscoSB/ukp.aspx?vw=1&docid=0019598901484802accffb23cd1cba8f_Remote_Access_Dial_In_User_Service__RADIUS__Server_settings_.xml&pid=2&respid=0&snid=4&dispid=0&cpage=search
The admin guide says:
Enable RADIUS Accounting—Enables tracking and measuring of the
resources a particular user has consumed, such as system time, amount of
data transmitted and received, and so on.
If you enable RADIUS accounting, it is enabled for the primary RADIUS
server and all backup servers.
I am not sure of the details, or format of the accounting info.
Hope this helps,
Dan