To be able to do the following trick you must at least be Pro level. While in Free Drive mode press Start and select 'Editor' and make anything (just skip). Then at the map section make a small race that takes place wherever you need to go. Even though there is a cross through a circle, you are off the road then.
On the south east of the island there is an acceleration race for cars that you unlock at pro level. It is called 170mph in heavy traffic. Start by going right to the left of the road and accelerating down the heavy shoulder. If you have a fast enough car you can do it in 30 secs or less. For reaching 170mph you get 15,000$ the just finish the race and retry and you money will keep going up!
There is a house called the wailupe beach house. After you exit the showcase thing of the home and you should be facing the way the road goes then you go 2.9 miles and STOP. Then go to "options" then to "online" then to "allow online interactions and hit "x" then "a". Then you wait for a racer to come up and challenge him/her and you race him however many times you want, but each time you cut across the grass to win in 5-6 seconds.(you set the finish line near you but across the road) ( 1 race = 20grand 2 races = 40grand etc) But in order to get the $ you have to do a race called pure acceleration and then you will get all the money you earned! I Have gotton 452MILLION Off thiss so far.