Test Drive Unlimited

There are lots of cheap (under 200K) homes that can be purchased in this game. The best way to rake in cash is to win Class A events, which consistently pay off in the $75 - $100K range. With a good Class A vehicle, you can make lots of money quickly.

Courier and vehicle delivery missions are a great way to make money too, and most of them don't take very long. Getting 100% on a vehicle delivery mission can earn you around $135 K. Most courier missions pay off around $40 K.

A couple of other good ways to make money:

1) There's a Class G Speed Challenge at Punchbowl Crater (near the bottom of the map) that pays $50K each time you win. The trick is to not follow the actual route: at the start of the challenge, move the car forward just enough to make the "Start" banner go away. Then turn around, and when you reach the stop sign, turn right and hit the gas down the long straight stretch. After you reach the 125 MPH threshold, press A to end the race. Then press Y and repeat the 30-second process as much as you like. Use a good G-Class vehicle like the Lamborghini Miura or Jaguar Type E Coupe with Level 3 upgrades.

or if you have some free time:

2) Halfway up the left side of the map along the coast is the Millionaire's Challenge, which pays out $1,000,000 each time you win. This race will take about 45 - 50 minutes to complete, and if you use a fully-upgraded MV Agusta F4 Tamburini motorcycle (available in the Ducati bike shop) it's not hard to finish within the 60-minute time limit.

Simply purchase 15 houses. All 15 need to be owned at the time to get achievement (e.g. no buying and selling the same house 15 times).

To get enought money for buy all cars and houses or smth else u can use my method. Simply got the fastest car u have and start winning speed challenge calling "180 mph in heavy traffic" Wahiawa which give u 70k $. The trick of this FASTEST money boost in this game that's u can win this challenge in 20 seconds or u wanna more quickly just turn back on start and that's will be 13 if ur good.

NOTE: requires driver lvl Champion (hightest)

To make money fast, you can try this:

Once you have achieved Champion Level (Gamerscore of 450 or more), you will unlock the Speed Race "180 mph in heavy traffic" in Wahiawa. If you look at the map, it's right where the mountains meet the woods, around the center of the map. If you're having trouble, use the filters and look only for speed races.

Get you best car (I used a B Class Ferrari F430, Tune 3). When the race starts, turn left and do a U turn. Stay on the right shoulder (not right lane), this way you can dodge incoming traffic and pass cars on your lane without moving so much. Stay to the right of the shoulder line. You will get to 180 in no time. Rinse and repeat.