TurboTax is NOT set up to report the sale of a vehicle (or other asset) that has varied in business percentage from year to year. So you should not be reporting the sale in the vehicle section and that is why it is coming up with weird numbers.
After telling the vehicle section you converted it to personal use, you should report the sale in the "Sale of Business Property" section. You would then report it as Costing $500, you took $500 of depreciation, and sold it for $100.
