NB, there
are sub-$100 feature (and even Android) phones available, though there are compromises to be made. Jose Briones's Dumbphone Finder lists about 45, sorted by price, and that's just what he's reviewed. Going to $250 there are about 65 options.
<https://josebriones.org/dumbphone-finder>
Once you're into the Android or Android-adjacent OS territory (LineageOS, SailfishOS (the Callback runs this), /e/OS, iodeOS, etc.), prices approach flagship mainstream Android or iOS devices ($600+ generally), and GrapheneOS specifically requires Google Pixel, at least until the Motorola partnership bears fruit.
And yes, scale of production and the need to be self-supporting rather than relying on business partnerships, advertising, and surveillance capitalism does tend to incur some price premium, though it's still quite possible to find affordable options.
I'd strongly recommend taking a look at Jose Briones's Dumbphone Finder (mentioned and linked above), his website (<https://josebriones.org/>), Substack (<https://josebriones.substack.com/>), and YouTube phone-review channel (<https://inv.nadeko.net/channel/UCFtVwG0NFd6gT3TXfMCU7oA>) in general, and /r/dumbphones on Reddit for more information. I'm going to write a longer top-level comment summarising the current state of my own research into this topic.
There is a larger community oriented around alternative mobile devices including more reviews and technical information. Given that manufacturers often obscure rather than clarify features and capabilities, this is often a preferred source.
Edit: Corrected device support, originally mis-stated LineageOS as restricted, rather than GrapheneOS.