The Chromecast Audio was a wonderful device. I unfortunately lost all mine in a move but there’s been absolutely no way to replace them :-(
In the US, MSRP at launch in 2015 was $35.
With inflation, that's worth ~$46 today.
I see one on eBay right now (open-box, allegedly never used) for $60, delivered, in buy-it-now format.
$60 is not an ideal sort of price, but it's not something that seems extortionist or anything compared to the inflation-adjusted price from 2015.
A self-updating hardware/software combination from Google can never be a long term solution. Reliance on old units from eBay (or not!) cannot make that situation any better nor any worse.