I used to buy the Blue Book every year, as a rough price guide. I knew of the criticisms of their valuation accuracy, and back then, they were almost always well up on the high side. That even took into account their notice that prices given should be considered full retail. Of course, I had to point that out to a lot of gun show vendors who pointed at their own copies, but also frequently priced their 65% gun as 90 to 100% condition.
After a while, I realized that a two to three year old copy was usually more accurate and closer to reality than a brand spanking new edition. So I started saving money by buying unsold older editions. By then, the price of new editions had also increased significantly.
The example Brian gave for current valuation of N.I.B. condition pre-64 Model 70 Featherweights was unbelievable. Any neophyte would instantly realize that collectors would pay a big premium for the scarcer .358 Win. vs. the very pedestrian and common .308 Win.
I suppose I'll just continue my present course of watching actual completed internet sale and auction results to get an idea of average buying and selling prices to expect.