With the caveat that I'm not actively watching ebay, and haven't really for several years -
I'm not surprised by this at all, and, I suspect we only have a couple years before early Intel machines do this too, so if anyone thinks they have an early Intel machine they'll want to have gotten cheap a few years ago, just go ahead and pick it up today.
It seems like the value of old tech follows a sort of a trough curve.
Something is built and then it's valuable as a new/fast/current/modern piece of technology with a working life ahead of it.
Then, the piece of tech does its working life and around the end of that working life, its value drops to zero pretty quickly.
The value stays at zero and there's this period where the thing is too new to be interesting but also more or less too old to be useful (The Great Plateau is making this phase a little weird because a lot of stuff, at least as far as generic x86-based computers sold for Windows/Linux/Unix go, from 10+ years ago still competently runs current software versions). (Macs also get weird here mostly because there's only one official/supported vendor for Mac hardware, and so you have Macs from 2013 that fetch 2-5x what a generic OEM or self-built system from that time will.
And then, during that low period, people toss things because they take up space and they can't really find people to take them.
After enough of that has happened, people start having nostalgia for that era and want machines, but they're a little difficult to shake loose and prices start rising.
The older a piece or category of tech is, the smaller all the numbers are, in terms of quantity, so it'll be interesting to say how this plays out long-term when we get to the point of, IDK, Mac OS X 10.6 on 2009-2011 Macs being legitimately "vintage" - especially because this was by the time of MacBooks being among the single most popular laptop models in existence.
The problem there with finding "good" examples of MacBook/Airs in 2030 is going to be that most of the people who bought and used them used them hard for a very long time. (Though, to their credit, since sometime in the mid-late 2000s the build quality overall on Mac laptops took a huge leap forward from where it was for the '90s and early 2000s, due in part to material changes and in part to Apple just getting better at it. (And, the much much greater popularity means that unless the number of people engaging in vintage computing as a hobby scales, even with greater failure rates and more difficult to recover from failure modes. (Especially dependent on what happens when the first person decides to just pull the battery out of a newer MacBook Pro and run it that way, and how that'll end compared to doing it on one of the older MacBook Pros.)
TL;DR - if we're noticing a raise in powerpc mac prices, I'm not surprised.