Becoming a billionaire in GTA V (not GTA Online)
Solution 1:
I haven't had much luck trying to destroy assets to affect stock prices either. But here's what's been working for me so far. It's not a guaranteed quick-billion but over time you'll get there.
Invest in Debonaire!
The stock price for Debonaire consistently fluctuates between around 4.8 and 7.4 (at least from what I've seen in my ~60 hours of story mode gameplay). Since the price is so low, that tiny change is actually a significant percentage. If you invest everything in the stock when you see it anywhere below 6 or so and then sell it back any time it goes over 7, you're making a decent percentage (up to 54%!) back on your investment at a relatively frequent interval.
If you are actually trying to reach $1,000,000,000 then this method is going to take a long time. There's probably something better out there. But at least it's something reliable to do until someone finds that better thing.
As for precisely how long, given your starting balance of 26 million and assuming an average ROI of 10% (YMMV) and ZERO extra spending, you'll have to invest and sell everything 39 times to reach a billion dollars. But only 19 times to afford the golf course.
Solution 2:
Like you, I didn't know all of the stock manipulating tricks until after I completed the game. It bothered me enough that I replayed most of the game in order to finish with over $2 billion. If I knew then what I know now, I wouldn't have started over. Not because there is nothing to spend the money on but because I have found a reliable method to accrue money after finishing the main story. Although there is a cap on how much money you can have at any given time, there is no such limit to the amount you can invest in a given stock, so long as you don't go over the $2.1 billion mark in shares of a stock you own.
At the moment, I have about $320 billion invested in BAWSAQ stocks. Why would I need that much money? The zombie apocalypse of course...dummy.
Anyway, almost all of this was accumulated in the same manner. Although it is claimed that the BAWSAQ exchange is based on the activities of online players, I don't believe it. If you watch the bawsaq exchange long enough, you will see fairly predictable trends in certain penny stocks. You will notice that certain stocks drop substantially for a few days and then rebound. Focusing on 3 or 4 penny stocks can net you 50% to 200% returns over the course of a few days. Every once in a while, you will see a stock go from $2 a share to $15-20. I'll take a 1000% return all day.
I've also found that designating 8-10 save games slots for different point in the trading process come in handy for those times when you were wrong about a stock.
I got tired of typing my original reply, but I had a good night's sleep and my morning coffee, so I'll add a few tips:
Stick to stocks valued at $0-4 a share. It is much more likely that a stock will jump from $4 to $8 than it is to jump from $50 to $100... at least in GTA.
Stick to BAWSAQ, it is much more volatile in the short term, which is what you want in this case.
Snap photos with your phone (real, not in game) of stock prices on a daily basis for a week or so. It really helps with identifying trends. Don't pay any attention to the 5 hour highs and lows. They are garbage.
Finally, I noticed that although certain stocks tend to follow a pattern, that doesn't seem to be the case in the long run. They seem to go up and down over the course of a month or so and then the pattern changes. For this and other reasons, save often and save with different mixes of stocks.
Thinking back though, I typically would roll back to a save point not because a stock tanked but because I sold it at a 30% gain when I could have sold it after a 60% gain. At some point, it isn't fun anymore. Trying to move 50 billion between stocks when the limit per transaction is 2.1 billion isn't worth it.
Solution 3:
Do not invest in Debonaire
Here's a simple answer: Firstly, do NOT invest in Debonaire. It's not special and will not keep going up.
Secondly, you need to get more strategic. Look for any stock that's less than $0.50. You are pretty much guaranteed a huge return with some patience.
When you find a stock that looks promising, go into the game and SAVE. Then put all each of your character's money into that stock, and then save into a DIFFERENT save file. (This will allow you to go back to your first save, should anything go wrong.)
(Note: The game can be a bit glitchy when trying to put ALL your money into a stock. Just double check that it's definitely left your character's account, and if it hasn't, try a slightly lower amount. That usually works.)
Then, remembering what you bought the stock for, keep checking the stock regularly, and when it's gotten close to the height it's reached over the past three months, log into the game and sell everything.