DMV usually uses book price if an owner does not know the price paid for the vehicle or have proof of the sale price (bill of sale, etc). The listed price can also be determined by the insurance statement of the value of the truck.
If the insurance lists more than 1500.00 (in error) this could be the problem. Most insurance agreements will only pay that listed price in case of a claim. The new owner should make sure that all figures match.