This is a great example of why you can't earn a billion dollars without doing something immoral.

First of all, you don't know how to build a house, you have some vague idea of it, maybe even some innovative one, and hire people who actually know how to implement that idea to do it for you, but you pocket some of the money because they don't know the exact worth, and because they need to live and no one else is paying them.

Then, what happens next is that some competitor springs up, and they want to poach your people by offering to pay them $475k and only keeping $25k in profits for themselves. You can't have that, so you start paying off government officials to deny or delay their building approvals, paying off mafia thugs to sabotage their construction or threaten workers who try to leave, you sue them for breaching worker non-competes or IP rights on the construction methods, etc.

All of these are part of how the construction industry actually operates, it is well known as one of the most corrupt industries around. Let's also not forget that the value of real estate is location location location - house and business prices are hugely determined by where the plot of land is, much more so than any differences in construction prowess, if we disregard the very top of the most complex projects ever made (of which there are way fewer than 20k).