"Book" value doesn't really matter much in the all-molded world. Perhaps insurance companies use NADA in the case of a totaled trailer... and then the fight is on!
Every single trailer has two values. What the seller is wanting to sell for and what a buyer is willing to pay. It's up to the two to come to an agreement. Location pays a big part too! For instance, while a beautiful 1968
boler may sell for $15,000 in Los Angeles, it may only sell for $12,000 in Vancouver, BC.
Isn't your Bill of Sale good enough? Otherwise see if NADA has a listing for your trailer. Good luck