You know the fun thing is, something like the Allwinner A133 - which is one of the most popular SOCs in lower-end tablets today - is like $5, or $3 in quantity.

It turns out it's actually not as hard as you'd expect to whip together your own board with one of those + LPDDR4 RAM + eMMC storage + fixings, and get yourself something like what you're talking about for... I dunno, sub $50? Maybe even sub $20 depending on how much RAM you put on it and what other capabilities you give it.

I'm in the middle of designing just such a board right now. Totally recommend taking a stab at it if you have any EE chops at all (or want to learn!)

Lets just go with $50 and $20. If you're looking at that on top of the cost of a raspberry pi, comparing that to a super low-end Android phone, used, for something like $80-$100, is that really the way to go? The OS is different but termux has enough features, especially after rooting, that you can probably run whatever you're shooting for. Of course as a hobby, the parts that you find fun don't have to be the parts that I, or anyone else finds fun, so don't take this as me pissing in your cereal, it's more like there's the milk part and the cornflake part and so different strokes for different folks.

Interesting. I'm currently having great fun learning systems programming on the Allwinner A64, and never considered the option of building a board with one, assuming they are still available. Are you documentating your project somewhere?