I've finished all the story missions, and most of the side-quests, but there's still several properties I can't afford yet. Some of these properties cost tens of millions of dollars, but I can't find a way to quickly earn that money. I'm aware there's always the option of playing the stock market, but that's a crap shoot after all of Lester's assassination missions are done.
Are there any reliable ways to quickly earn the money needed to buy the more expensive properties? Do the options differ for each character?