Price guides whether it be chips, stamps, coins or used cars are exactly thst G U I D E S.
There will always be more recent sales, finds etc that will skew the prices in any guide.

Buy a diamond for $10,000 It will usually be appraised for at least twice as much, then try and sell it and find what it is really worth.

I owned 1/2 of an art gellery once selling fine prints. Publishers used to print price lists to dupe purchasers into thinking their "investment" was escalating at a rapid rate until they were hit with reality and found that their purchases were now selling at 1/2 of what they paid.

To me a guide is a good working knowlege period