Do you know about the DTM trick to pile up big bucks in a hurry? If not, here it is:
Run the second Special Condition (Easy) race twice and sell the two Toyota rally cars you'll win. Go to Germany and buy an Opel Astra Touring Car. Buy the Stage 3 NA upgrade. After an oil change you'll have 654 HP. Use this chassis setup:
http://www.gtvault.com/SetupDisplay.cfm?SetupID=0202J844508E887CA06240441
Use all those settings, but also turn the TCS up to 7.
Now go to the European Events and enter the Astra in the Deutsche Touring Car Meisterschaft (DTM), or, in English, the German Touring Car Championship.
B-Spec every event. Set the speed at 4, and leave "Overtake" on. Click R1 to display the map, then click and hold L1 while you D-Pad right to change the speed to x3, or three times normal. Each event will then only take about three minutes. If you choose to monitor things, you can blow through it all in about 17 minutes. When the prize money starts being awarded, press X or Start to speed up the process.
The prize car's resale value is 743,749. You get 15,000 for winning each event, and 75,000 for the championship, so your total haul for less than 20 minutes is 893,749.
Best of all, you can do other things while your B-Spec driver is piling up cash for you, like eating dinner or sorting socks.
Repeat early and often. I like to go through it three times while I'm getting ready for work. That's over 2,500,000 without even playing. At that rate, you can collect a bunch of very expensive cars in a hurry.
(You can use your pile of cash to get an Audi R8 or an '03 Pescarolo C60. The '03 C60 gets great tire life, giving it a big advantage over other Prototypes. Leave the suspension stock, turn off ASM, buy Stage 4 and R1, R2, and R4 tires, and have fun.)