Raspberry Pi is based on smart phone chips, very specific chips from one manufacturer. Raspberry Pi Foundation is not the main customer for this manufacturer and chips used for Raspberry Pi are not their only product – and now, during the big 'chip shortages' and supply chain problems other customers and other chips are given priority. There are no (or not enough) new chips for Raspberry Pis so there are no new Raspberries, so availability is dropping and prices are soaring.
I guess the same is true for most other SBCs.
For my hobby projects I switched to Raspberry Pi Pico. It is not a SBC, you won't run Linux on that, but it is a very capable microcontroller board which is enough for my needs. It is way cheaper much more available. And I won't look back – it occurred to me that things are much simpler when there is no whole OS on my devices and everything the device does is in my own code.
There are no problems with Pico availability, as it is based on a simpler, custom chip, designed by Raspberry Pi Foundation and manufactured for Raspberry Pi Foundation – they are no longer dependent on a single supplier.