Gran Turismo 4

Hm... Quickest way to make money off the start would be to do the B and A licenses yourself (do not transfer from GT3), take the Lupo 1.4 won from the B-license to the Sunday Cup, win all the races, and sell the Abarth prize car. Go to one of the used car lots and pick up basically anything with over 220hp or so, but there's a few that you should be aware of as excellent cars. My personal recommendation would be a 1998 or newer Mazda RX-7. Anyhow, take your new car to the second event in the "Special Conditions Hall" (Bottom left corner of the main GT mode menu), called Capri Rally. Win both races and sell the prize car for a cool 265,000 or so. Go back, do it again. When prompted to "clear previous race data", do so. Do both races, get the price car, sell it.

Now then, that's the quickest, easiest way to make money quickly early on... So if you ever really need money, do that unless!

If you have a touring car (I recommend the Audi TT-R ABT touring car), either run the Touring Car championship yourself in A-spec with it stock, or add a turbo and let B-spec Bob race it all. The championship itself if you win all races is good for a decent chunk of money, and the prize car is worth a nice 743,000 credits or so. Simply wash, rinse, repeat. If you plan on having B-spec Bob drive, a stage 3 or 4 turbo kit is not a bad idea. Also, to shave time in B-spec, press R1 to go to Race Monitor mode, hold L1 and press the right directional pad three times.

To my knowledge, those two are the quickest and most effective ways of making money, and therefore, this question should be pretty much answered.