that value is undercut by its inaccessibility

You are saying this like GrapheneOS only runs on some unobtanium hardware. I can literally hop on my bike and pick up a phone that runs GrapheneOS in 5 minutes, every day of the week. Also, it's available in pretty much all price classes except maybe a 100-200 Euro phone that runs on a Unisoc CPU. Pixel 9a regularly goes for 350 Euro here and you can go all the way up to an expensive flagship with a Pixel Fold (or anything in between). Even in the 100-200 bracket you can probably pick up a refurbished 8a that should still be supported until 2031.

I know that they are not sold in every country, but worst case it should be possible to get your hands on one second hand or refurbished.