Sales Tax on a Private Vehicle Sale

I'm about to buy a car for the first time in Florida. Every time I've bought a car and paid the sales tax it's been a percentage of the sales price. I'm buying an older car from a family member through a private sale for $500 (quite a bargain). My MIL said they recently changed the rules so that sales tax is 6% of the value of the car, not necessarily how much you bought it for. Does anyone know if this is true? If so how do I calculate how much it will be?