I had a few million already at that point, so instant cha-ching. I'm going off memory, but the odds were usually anywhere from about 30-1 to 60-1, I recall it took me about 6 or 7 reloads before I got the big payday. If he doesnt, reload save game, try again. Gather up some cash, bet on the long shot to win, if he wins, payday. In GTA San Andreas it was very easy to glitch your way to mega millions with the Off Track Betting scheme. A quick theory as to why the horse track has not been present in some time (was it in GTA IV? I honestly cant remember)
