There can't be any faster way of making money than Gambling at one of the Inside Track betting places...
I was very skeptikal of what people were saying about betting on the horse with 10/1 odds, but after a while (maybe 20mins TOTAL) i had gone from $90,000 to 8 Million dollars.
I kept reading people saying that thinking they wanted to have a joke on you losing your money... but it actually can work...
I bet a few $5000 races, and lost about 3 or 4 ties in a row, but as long as you have enough money to make large bets, then betting $50,000 on the 10/1 or 11/1 horses really really pay off...
If you have Catalinas' house save point, there is an Inside Track betting just down the (dirt) road, and shouldn't take you more than 20 seconds to drive there. If you win, go there and save, if you lose, reload from that location. After about 5-10 trys/loads, i managed to go from $90,000 to 8 million.
PLUS, after completeing Zeros' missions you collect a max of $5k from the Dollar sign outside his store.