Here a few ideas, well for starters stunts are an easy way to make a lot of double RP I know your thinking: ok l already do that. Furthermore, do you know of the Blazer Aqua- Water Slide stunt race. OK if you don't know of this method I will tell you to start on this stunt race and put it on one lap keep the settings the same and close the lobby.
You can get a humongous amount of money and RP for doing this, and you can invite friends if you want but make sure to inform them on how to do this as well. OK so, when you start off the race by yourself or with a friend what you do is you play the race as usual until you reach the green tunnel. Then once you come out of the green tube into the air turn your camera view with your stick on your controller or whatever console you're on. So you want to die from falling, you want to try and land safely as possible.
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Also, you will you get 400 XP everytime you do this, and you can repeat it many times as you want to. However, you can only do this 15 Times a race so when you do finish doing this make sure you just do it 15 times. Be cautious and count, so you don't lose focus on how many times you have done it. Oh, yea once you passed the eight-minute mark at the end of the race you will be receiving 16,000 plus money as-well as 3.500 RP for completing that race.
Of course, you could do the math 400 x 15 and calculate how much XP you would receive, and don't forget the 2,000 xp your already going to get. If you haven't tried l suggest you do it's a pretty unique way of making a lot of RP and money.