Base conjecture was using the value of a good delivery gold bar to match up with the in-game gold bars you can acquire, which emulate the good delivery bar. He extrapolated that to find a real-world value for the in-game G, and concluded that there was some $500 million tied up in the mansion itself.
The method seems reasonable enough, but in my opinion the conclusion is not. He seems to have totaled up the value of individual items, but neglected that there's in fact a maximum amount of G you can get in the game - depending on your version, NTSC or PAL, that's either 142,390,000 G or 186,440,000 G. Using the conversion method provided ($36,819 to 100,000 G), you end up with either $52,426,574.10 or $68,645,343.60, respectively.
Thing is, this entire discussion takes place in the last two minutes of the video. The remaining 14 are dedicated to a discussion surrounding what the ghost mansion would be worth - which, as it disappears upon completing the game, seems to be a construct of the Boos and not explicitly a real, physical place, making the whole discussion kind of moot.
He does seem to address the fact that the heights of the Mario Bros and most of the things in the Mushroom Kingdom are subject to Lol Wat Standards and reused a height for Mario that he'd settled on previously (5'1") as the basis of the rest of the distance talk. Extrapolating from Luigi's run cycle to counteract the skewed camera perspective is a clever play, but I'm unsure if it's really the right approach; it would have to assume a lot of things about the kind of run cycle that Luigi is capable of and/or using. I mean I can run at multiple consistent speeds and cross the same distance in multiple times, but I don't know if the mechanics of doing so have some sort of inherent mathematical formula which could then calculate accurate, or even napkin measurements from it.
Much of the calculations are black boxed and much of the talk is probably irrelevant. However, in its defense I learned a few things I intend to research further in terms of architecture. I have projects. No need to talk about them.