The registration fees are based on the assessed value of the vehicle. They use tables for that and there's nothing you can do about it.

The SALES tax charged when you apply for a new title, and didn't buy from a dealer who's already computed or charged the sales tax, is based on the reported sale price of the vehicle.

If you put down anything other than the actual price you paid for the car, you're engaging in tax fraud, but the chances are slim that they'll investigate or that you'd be caught.