You DO reside in the USA, right? But you're saying that prices of guns aren't "like cars or other things?" Do you not buy "other things?" Gasoline prices fluctuate daily. So do prices of produce, dairy products, coffee, and just about anything that has to be manufactured requiring the use of petroleum.
You're taking it literally as to mean everything else. "Other things" doesn't mean everything. You kind of made my point for me. If the prices on the items you mentioned fluctuate regularly, why in God's name would you buy a static book aka a snapshot in time on what those prices should be?
I'd submit, that in normal times (e.g., not when a state legislature is voting on new restrictive legislation on firearms), gun prices, as well as ammo prices, are fairly stable, compared to most consumer goods.
I'd say that state and federal legislation scares, mass shootings, low supply from manufactures, discontinuations, release of new generations, ATF rule changes, import bans, etc have respectively become a routine thing which all can have an affect on the price of firearms and ammo.
Some firearms prices maybe stable like Glocks or the like, but the prices of many other firearms (not all) fluctuate regularly depending on several factors. Even if you look at most online retailers' prices, they even change from one day/week/month to the next. I can't even begin to tell you how often I put a gun in my online cart, procrastinated on buying, and when I came back a day later the price went up. The price firearms can be found for NIB will affect the used pricing to some extent.
Then you also have to take into account whether you're selling online or locally. The going price of firearms in CA, NYC, DC, etc aren't going to be the same as in Texas or Utah. Does the book that into account? Online prices on used or new guns tend to be cheaper than off line prices as well in my experience.
The availablity of the Colt Python wasn't equal to demand. The MSRP was $1499 or something like that, but the market at that time was well over MSRP. Now you can get them under MSRP. CZ often manufacturers some of their pistols in batches (HK does this too with some imported models), and they regularly abruptly discontinued certain model numbers. There has been times when Colt took on a big military contract for ARs, so they temporarily stopped or decreased production to civilians. Prices for Colt parts and rifles shot up.... You get the picture... This all affects demand and prices. Depending on when you purchased the price book, the suggested pricing may or may not be way off.