wireless computer mic for classroom lecture capture?

Hi folks,
I need recommendations for a microphone that I can begin adding to some of our higher ed model classrooms so our professors can begin to capture their course lectures and produce podcasts (or enhanced podcasts) of them for the web.

It needs to be a wireless lav-type unit so they are free to roam around the room while lecturing.

A friend suggested going w/ a USB bluetooth headset which sounds cool but I am hoping for a lavaliere-type mic since "who wants to stick a bluetooth headset in their ear after the previous professor had it in his/her ear and hour ago?"

Any suggestions would be appreciated. I'd like to keep these mics under $100 or so.