The sale price of the vehicle you traded in would be the amount they reduced the purchased vehicle by. If your new car is $50,000 and you paid $40,000, the trade in is $10,000.
Sounds like you have been taking depreciation as opposed to mileage. The traded vehicle is probably fully depreciated so you would have a $10,000 gain on the trade-in. Then you can depreciate the new vehicle from $50,000.
