Regarding the need for the section III preparation materials, I'm speaking from a non-science background and have found that other students with scientific degrees that aren't "straight" chemistry or similarly focused disciplines also needed the preparation material.
I did try getting standard text books from the library and to be fair that might have worked if I had known exactly what topics to revise, but the only way I can see of really pinpointing your revision is through the GAMSAT-focused preparation materials.
I agree that the GAMSAT, whilst it needs to shed some irrelevancies, does give you a much more vigorous grilling than the UKCAT, which I too thought was a narrow indicator of ability. And at least the GAMSAT does give students some credit for hard work.
True it's hard to speculate on their profits (who knows how the company is structured and what they get paid), but I'd be very surprised if there wasn't a significant cut taken somewhere. if 3000-4000 people take the exam in London that's between £495,000-660,000 with £30 VAT taken off. It's largely computer marked, with student invigilators, but sure, I wouldn't know.