Doc brown tells Marty he nearly spent his entire family fortune making the time machine. So how could he afford the nearly $40,000 hover conversion to the time machine ? My guess is that he did the opposite of what he tells marty. He went into the future and gambled on world series games. He did tell marty that he was going to be able to see 25 years of world series winners. Plus, he has so much money when they go back to the 50's in part two. So, is Doc Brown a hypocrite in telling Marty that he didn't create the time machine for financial gain? It's just a theory.