the US dollar has gone down in value, you will have to do better than $20
i dont think i have ever found a webcam for that cheap new.
maybe try ebay for a used one.. just find some in your price range and go to the website of the webcam and see if there are drivers for windows 7.
maybe you get a better webcam than what you would get if you bought a new one for $20
just make sure the listing says the webcam works without any errors.