I just learnt about the Nokia 105, which was apparently just 15€ in 2013 and has had three rereleases because it was such a popular phone
thanks to @whitequark for showing off the extremely cost-saving board
btw, the 105 is very much a successor to the Nokia 1100, the original cheap-but-rugged phone, and the first phone to have a built-in flashlight. The keypad might not feel great, but it's dustproof! it's one of the most popular phones ever (over 200 million sold!)