It's because you jeff must have had other soundtoys licenses on your account. With a single V4 license like mine, it's 199 dollars. With a couple other plugins including the free ones they released as a limited offer or something, you can get down to 50 bucks.
Like with the "microshift" etc, you can save a dozen dollars from the 199 (they justify it as a thank you for following them eventhough the plugin was given), with a standalone plugin + V4 maybe another 50, down to 50. In my case with nothing else than V4, it's 199. Also once you accept you're giving back all the licenses you tick on the page (otherwise you would still be able to sell them eventhough you profited from them to get a discount)