Hmm. As you mentioned, you can get developer boards for less than $50. But, does your demographic have HDMI televisions to connect them to? Keyboards? Storage?
For keyboards, we can deal with refurbished or used. For storage, you are right, SDcard is the primary option, in a standalone product, it's not cheap (about 20%) comparing the total board price but we can find a more suitable formula if it will be included within the board. But the display will still a challenge if we choose to build custom board. HDMI TV are not such used in rural zones, they still need some time to be accessible in price and popularity.