From what I read so far, all the OEM mics are very similar (if not the same). If you can find an Audi with OnStar or a B5 with the phone kit, the mic should have the same part number. I don't think non-OEM mics will work that well, based on what I read so far.
I can understand you wanting a cheaper mic, but really you will be dissapointed with anything but the audi one. The mic is designed to work with the electronic control box, the noise cancellation is excellent, so why compromise?