Expecting a 50,100,150 $ card to perform as something that cost 5K+ ?
No, i don't expect this, but....have you ever heard about wi-spy? 99$ device with simple spectrum analisys on it's wireless band.
I think it has a particular driver (or firmware function) wich bypass the layer 2 chipset on the card, and talk directly with layer1.
The card report the amplitude of the received signal with 1MHz resolution (but the new version has higher resolution, about 100kHz).
I think this is not an impossible thing for developers...don't you think?