Ask HN: How hard would it be to make a cheap, hackable phone?

What I've wanted for years is something less extravagant and more mass market than the Neo900. The Greenphone was a reasonable example of this in its day, but nobody seems to be producing anything like it now for a price that is even slightly reasonable.

What I really want to see are mobile devices which can competently run vanilla Linux with a minimum of blobs, where no major company dominates the ecosystem the way that Google does with android.

Hardware like the raspberry pi, CHIP, etc is all dirt cheap, so I'd be surprised if this couldn't be done for (at a guess) $75 with a soc, screen and case if volume was sufficient and specs weren't significantly higher than those devices.

Perhaps I am naive and this can't be done, but I increasingly see no reason why not. Am I missing something? Is this a real, unfilled niche?