Select the fair market value of the vehicle when you changed the use (used it for business) as the addition cost.
A vehicle that costs over $30,000 is class 10.1 and is limited to 30,000 plus tax.
A vehicle that cost less that $30,000 is class 10 so you enter the total cost including taxes.
This is all done in the auto expense page near the bottom. (CCA or Capital cost allowance)