I think you're all focusing on the price / quantity for the wrong reason. In no way am I saying that you should swap out your $5 MCUs for $1 ones to save money at 100 unit quantity production run. Obviously that would be ridiculously stupid.
It's more like, "hey, what's out there these days, in 2017? What do all these manufacturer's development environments look like? If you're starting a new, long-term project, what parts should you consider?"
I picked "100 units" because it's sort of where pricing trends start to happen. It's a quantity that professionals and hobbyists can both wrap their heads around. And it's also small enough that it avoids shopping / negotiating directly with an MCU vendor. Look, last time I did a high-volume commercial product, I called up NXP and told them what I wanted to pay for their MCUs. So looking at quantities of 1k, 10k or 100k gets more challenging, because it really depends on your relationships with the vendors.
And I picked $1 price point because without a price limit on this comparison, it's overwhelming to whittle things down. These days, $1 buys you a mid-range, general-purpose basic microcontroller that's got dozens of I/O, half a dozen or more PWM channels, 10 or 12 bit ADC, decent sets of timers, and enough flash and RAM to cover most general-purpose entry-level needs. Personally, I think limiting price to $1 is a great way to give people some intuition about these families — and if you need more, you can always shop up in price, as most of these families are quite large, and vary quite little as you get up into the $10 price (from a broad perspective, what's the difference between the STM32F0 ecosystem and the STM32F4 ecosystem? Not much)