The difference between good and bad webcams is more to do with image quality, dealing with light levels and so on - rather than just megapixels (resolution)
It's exactly the same situation as with stills cameras. It's not hard to find a phone camera with 6 megapixels, while my old D-SLR is "only" 6 megapixels.
But I bet you could tell which photos came from which - the larger, better quality sensor, as well as much better lenses make a world of difference.
Essentially - in good diffuse light many cheap webcams will be just fine. But as soon as you get to lower light, strong side or backlighting, movement and so on that's when the better quality ones show their worth. I'd always go with a known brand - something like a logitech one.
