If you've not done any of Lester's assassination missions yet, I suggest you start those. You do still have to have Franklin, and it's ideal to have all 3 characters. Why? Well, with each of the (I think) 5 missions you're basically destroying companies by killing people associated with them. For example, in the first mission, before you start, open your phone's internet and click money and trading, then BAWSAQ, then find and select Betta Pharmaceutical. With each character, you should have $25+M if you complete "The Big Score". Make sure you haven't completed the story yet. With each character invest all your money in Betta Pharmaceutical and after saving twice in a row, you should've roughly doubled your $, so sell then because the stock will crash HARD shortly thereafter. For the last assassination mission, the construction assassination, invest in GCD, or Gold Coast Development, again with all 3 characters, and using the method described above.