The most fun I ever had for $20 in a car was an SCCA autocross in 1999.
I would LOVE to build a car just for this purpose.
Sadly, I traded this in, October of 2002 for an SUV to pull the Camper with. The service manager at the dealership I sold it to, still has it. It wouldn't be my last BMW though! See the Winter beater 1987 325e.
Past Project: 1964 Chevy Nova