There are other, less extreme fads. Certain early transistors, for instance, command high prices in the right quarters. Old radios come in a galaxy of styles, shapes, and sizes. Some people make a living by restoring and reselling old radios. Again, it's pointless to get mad or call this gouging.
Antique dealers are in business to make money, which they do by buying cheap and selling dear.
Actually, there are. You can buy collector books that list values for thousands of radios. And, despite all my caveats above, if you're interested in collecting, I'd recommend that you get one or more of these books. And they can teach you a lot about the relative values of different categories of radios. The European guide La Grand Livre de TSF uses relative pricing, with two or three dollar signs representing an average value, and five dollar signs the highest.