One possible way to get them closer as cheap as paperback books might be to make them per book. Like, if you didn't need an OS or networking or anything but a charging port, because the thing only ran one app and came preinstalled with one book, you could theoretically get the cost way down.
The only market I can really think of for these would be airports, or maybe libraries, for something you'd borrow and give back shortly thereafter. But then I don't know how they'd be enough of an advantage over paper books to be worth it.
I'd had to be like lending a book or renting e-scooters; otherwise it'd turn into a huge e-waste issue.
This makes me think of Star Trek's PADD - i.e. an iPad imagined some 20 years before the first one was built. Unlike real iPads, PADDs were used more like pieces of paper or individual books - people would exchange them, they'd submit reports to their commanders by leaving a PADD on their desk; one could easily have half a dozen of those PADDs spread around when researching something, etc.
There's a lot of merit to that idea - 15 years since iPad came out, it's becoming clear that a single expensive screen for everything is not a way to go, at least not for productive work. But I feel that, beyond dirt-cheap hardware, it requires a closed organizational ecosystem.
On the show, it's plausible. A starship is like an office building (except people don't live it for months at a time); PADDs can circulate freely around it, going from person to person, changing purposes along the way. Some get lost or broken and need replacement, but it's a neat, closed system of same-ish hardware in constant flow. But try to do that as a regular consumer product/service in the real world, and it stops making as much sense. It's a problem similar to various tool sharing initiatives - they technically exist, but they aren't really "a thing". People prefer to own stuff to ensure it's available and in working condition.
My kid was recently gifted a Yoto Box which plays aIndio and he loves it. I can’t help but think that putting ebooks on scannable cards might be a seriously underrated form factor. At that point you don’t need a file browser, networking, or anything. You just need to be able to read text and images off the card.
The Yoto itself does need networking because the card is just an NFC tag that points to a file in a database somewhere that the player has to download. But something similar to a video game cartridge instead could probably work. IIRC a Nintendo Switch game cartridge costs about $10 per unit and in this use case you wouldn’t even need them to be that small so you could maybe get them cheaper.