@StPark -- yeah I think someone isn't telling the whole story.
www.cdtfa.ca.gov
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Determining the Amount Subject to Tax
The total purchase price of your vehicle is subject to tax. The total purchase price includes any type of payment, such as cash, checks, the payment or assumption of a loan or debt, and the fair market value of any property and/or services traded, bartered, or exchanged for the vehicle.
Example #1
You take over monthly payments for a car your friend can no longer afford and, in exchange, your friend transfers his or her interest in the car over to you. You owe use tax on the balance of the loan at the time you assumed the debt, plus any cash you paid for the car. You still owe use tax even if you do not make any cash payment directly to your friend and only assumed the unpaid debt.
Example #2
You purchase a vehicle for $5,000. As payment, you give the seller your current vehicle valued at $3,000 and $2,000 in cash. You owe use tax on the entire $5,000 purchase price.
Example #3
You trade vehicles with another person. No money is involved in the transaction. The vehicle you traded has a current market value of $5,000 at the time of the exchange, which is considered to be your purchase price for your new vehicle. You owe use tax on the $5,000 purchase price.
Example #4
You purchase a vehicle from a private party. The seller knows that you are a painter and offers you the car in exchange for painting his house. You would usually charge $5,000 for this service. You agree to the exchange. You owe use tax on the $5,000 value of the service you performed.
Credit for Tax Paid to Another State
If you paid tax to another state when purchasing your vehicle, you may be entitled to claim a credit for the tax previously paid to another state.
For example, if you previously paid $1,500 sales or use tax to another state for the purchase of the vehicle, and the California use tax due is $2,000, the balance of use tax due to California would be $500.
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they even have a fancy web application to help you calculate the fees/taxes
DMV Fee calculator for VLF, renewal, new vehicles, new residents, and used vehicles.
www.dmv.ca.gov