This seems unfair, but most states view cash rebates as a form of payment from the manufacturer and conclude that it does not affect the purchase price of the car.
So what this means is if you purchase a car for $25,000 and there is a $3,000 cash-back rebate, you will be taxed on the full $25,000 before the rebate is subtracted. In a state like California which has 10% sales tax, that's an additional $300!
It's good to keep this in mind so you're not surprised when you see all the final numbers. It's not uncommon for car buyers to accuse dealers of fraud when this happens, but it's a legitimate tax. Thankfully, there are some states that don't tax cash rebates.
States That Do Not Tax Cash Rebates