That's not built with just any rock. Cost depends on the type of rock - we used a lot of granite in the pond because it was significantly cheaper than Missouri moss rock that we used for the waterfall and edging.
But I agree - it's not $500K if you can do it yourself, but that's not exactly a DIY pond for the average person.
Agreed it is not just any rock but a contractor that buys thousands of tons of rock a year from a supplier can get any rock cheaper.
The price of different kinds of stone isn’t that different, it is mostly where they have to go to get it, is the biggest cost driver. Middle men drive up the cost ridiculously high, as opposed to getting it directly from the quarry.
Also as you said, I am not a typical homeowner, since I can bypass middle men on getting materials directly from the source and have the heavy equipment to do the work. Even so, my opportunity cost alone on doing a personal project like that would deter me from doing it.
As MitchM said I would spend the 500k on waterfront property somewhere rather than go through all that.