I read a lot of threads where people said they didn't take advantage of the stock market during the main story, and they can't make enough money to buy the golf course etc. I saw a lot of people claim it's nearly impossible to get that kind of money if you don't exploit the stock market during the main story. That's incorrect.
I didn't use the stock market at all, and I'm at 96% completion. I finished the story with around $35 million per character. Then I invested $10 million in the Bank of Liberty stock when it was down, and made $1.6 million. That's a 16% profit. I did the math on this... If you start with $35 million, and only make a 10% profit each time, it will only take 18 or 19 times to reach $200 million. Sure, that's a little time consuming, but it's not that bad. And surely not impossible.
Knowing that, all you need to do is buy a crappy stock when it's really close to it's minimum value, keep checking it, and sell when you're going to make a decent profit. Rinse and repeat. It does work.