https://screenrant.com/why-doc-brown-rich-back-to-the-future/
In the Back to the Future trilogy, Doc Brown needed a lot of money to finance his invention of time travel. Luckily, he already had the resources.
I used to be a huge BTTF fan and from what I recall he inherited most of his money.
Interesting the other theories including insurance fraud.
The only indication we ever get of the cost is the phrase "entire family fortune".
That could mean anything.
In BTTF3 when they discover Doc's picture from 1885 in the library, Marty says maybe there was another Emmett Brown in 1885. This is where Doc lets Marty and the audience know that his family are the "von brauns" and they had changed their name during WW1. So maybe that family connection has something to do with their wealth. This would also explain Doc's love of science.
shareHe came from a wealthy family so he probably sold his estate through the years, which is something people usually doesn't do because the smart thing to do it is to pass it to your descendants.
Eventually he burned down his house to commit insurance fraud as you can see in a newspaper clip in the opening credits of the first movie.