Regardless of how cheap the seller says he sold it to you, at the MTO they have the values for the vehicles and they rate them as:
Low - Mid - Highest Value of vehicle according to the year/mileage they really don't care about condition of the vehicle or equipment since they don't take the time to leave their desks and see it.
So technically you can show up with a bill of sale for $1000 for 2012 car but when you register it they will look at the book value and based how much you are paying from that. They've changed that about 5-8 years ago because of how many people where showing up with a bill of sale of a brand new car for $1000 and paying only $115 for taxes. You have to be careful because if you say that you bough it at a super low price, they can send a letter to the seller asking them to confirm the sale price of the vehicle. And if he or she say different of what you told them they will come after you for the difference...