Well, to oversimplify if you have a 3MP fisheye camera, split into a quad view, you're essentially looking at four 3/4 megapixel cameras with 90° fields of view. Additionally, dewarping DOES affect quality, so it's not a direct comparison. No matter how good a dewarping SDK is, there is some loss of detail as some pixels must be stretched or squashed, and object geometry is often off, and straight lines skewed.
So four 720p cameras with a similar field of view would provide better pixels per foot, without the issues introduced by dewarping.
All that being said, if all you're looking to do is cover a very small area or see activity only, a $200 fisheye like this might fit.