I used to build race cars. Two guys I knew did this.
The first one was actually a massive nerd who by day worked for some shipbuilding company that designed and made composite hulls for the Navy. It was big money with few working hours.
The other guy owned some random worthless land in the middle of nowhere and some oil company sent him money each month for mineral rights.
So essentially both had limitless funds for what is usually a side hobby.
A common thing I’ve told people starting out is to find a class or race you want to compete in and build the car for that. It’s almost always cheaper to have one race car and one street car than it is to have a car that tries to do both.