go to Nam Cobanda Isle (it opens at the end of the game) and buy some chips there (about 400-500) to start off, and go play poker. Start with a bet of about 40 chips. When you win a hand, you get to play High-Low (this allows you to double your bet each time you get it right), which is basically, it shows you a card, and you guess if the next card will be higher or lower. If the card is 10 or higher, say it's lower, and if it's 5 or lower, say it will be higher. If you get a face card of 6, 7, 8, 9 cash out since they're kinda in the middle numbers and hard to predict.
Basically, just keep doing this until you have several thousand chips, at which point I'd increase my wager to 500 chips, which is max bet. In about 30 mins to 1 hour you can easily get several hundred thousand chips.
When you have enough chips, go to the prize exchanger guy, and buy 15 of each of the different types of rare gels there (there are 4). When you do this, simply go to the nearest merchant, and sell 'em for cash. This nets you 172, 500 per run. Easy money =)
360 GT: Echelon23 Currently playing: CoD4, Civ. Revolution, Too Human, Tales of Vesperia