you can't get a "good" microphone for under $20 ;-) (unless it's some kind of liquidation sale!) Sometimes, you actually have to pay for something. Try looking for a "usb podcasting microphone" (many brands) for $50 - $100 Sometimes a "good price" really isn't, because the quality of the gear is bad.
Comments
(I'm new to sound)
http://www.amazon.com/Logitech-981-000246-USB-Desktop-Microphone/dp/B002RL84OY/ref=sr_1_1?ie=UTF8&qid=1306630925&sr=8-1
Cheers, Weswog