I think what Kirk was getting at is that the purchase price of the book may often be saved on the purchase of just one strike, knowing the value ahead of time.

The estimated values given in most books, while not cast in stone, are a big help in determining which strikes to purchase when offered for sale.

Same thing goes for selling strikes. You wouldn't want to sell a premium valued strike at common strike prices. The various guides published provide that information and generally speaking are well worth the price paid for the book.