The cheapest flash microcontroller you can buy is actually an Arm Cortex-M0+
I'm not sure why people try and go for the cheapest microcontroller. These things all have unit prices in the single digit dollars or less in actual production costs. Finding a cheap microcontroller is entirely about finding a cheap reseller of whatever you're trying to get rather than finding a cheap part specifically. Only when you get to truly large volumes such that you can get volume discounts does unit price really start to matter.Especially if you're a hobbyist, price should just basically not be a factor in what you're choosing as they're all in the same ballpark from resellers.