Please don't take my comment the wrong way as sarcasm toward you I'm not trying to make you feel low, there are a bunch of of us feeling exactly the same as you over poor prize roulettes. No matter what hard events I do I get the same rubbish prizes too. They are usually just parts you can already buy.
In all the time I have been playing the game, I've won only one titanium piston rods for Clio Trophy '15, one weight reduction stage 5 for Supra GT RZ '16, and only one VR38DETT-GTR-NISMO engine for the 180 SX.
Everything else was carbon ceramic brakes for the radical, racing crankshafts for Maserati GranTurismo S '08, Clio Trophy '15, and a racing silencer for the Toyota 86 GT Limited '16. You can buy these though. Most of the time the prizes are below average, such as 2K, 5K mostly and rarely 10K - 30K payout.