Just bought a Nook Simple Touch from ebay for £9 delivered - I already have one that I bought new for £29 delivered back in 2014 - but a second one felt useful.
Getting an old but useable ereader for £20 or less is very doable right now - plus you can get the Nook simple touch to run (very old) Android.
I was so enamoured with the original gen2 Nook I acquired a pile of them. The batteries, unfortunately, are all dead and I've found no reliable replacements. Then there's that slimy rubber thing that happens after so many years, and that makes anything intolerably miserable to handle.
I sure did adore the old black n white Nooks, which I could load as I pleased with any file of my choice, often converting text and PDFs to ebook with Calibre.
AliExpress now sells the batteries - about £9 delivered. I haven't bought one yet (both my nook last over a day before needing a charge and that is fine for me for now) - and I still have a battery somewhere from my very first nook that I tried putting in my pocket then cycling to the station. Not a good idea, no idea how that battery is - I really should charge it to see.