Would it be possible to software process images taken by large numbers of amateurs of the same point in the sky to get even better results even cheaper?
Yes, in principle. In practice, there would be so much variation in the sensitivities of the detectors that it would be a nightmare to do the image reduction. In normal observing you have to do "flat fielding" every night (or at least every few nights) where you take an image of the sky at twilight so that you can calibrate the different sensitivities of the different pixels in the detector. Then all the pixel scales will be different (i.e., a single pixel on one telescope will correspond to a slightly different area than on another telescope), so you'll have to figure out how to scale all the images to overlay them all. It's just easier to do it all yourself.