I found a way that requires FAR less effort. The easiest way is to bet on cock fights. ALWAYS bet on the one with more wins, it's a guaranteed win. In the case that both have 0 wins/tied, just exit out of the betting screen and go back to betting. The most you can bet is 100,000, but if you win, it's an easy 100,000, and it only takes about 30 seconds. I kept doing this and easily got the most expensive car and motorcycle in a matter of minutes.
Yea i tried that but i bet 100k on a bird with 6 wins and the bird with 0 won, so it seems more random than that.