RP2040 should cost $0.70 in some quantity (https://www.digikey.com/en/products/detail/raspberry-pi/SC09...) and in low-quantity it's $1 from places like CanaKit. That's actually shockingly cheap for an ARM Cortex M0; I was assuming your ATmegas and PIC18s with a similar pin count (40, 48, 64) would be a bit cheaper, but they seem to be around $2-4 in similar low-production quantity. Power efficiency won't be anywhere near as good with the ARM part, but if you're making a battery-powered device, you might be able to use the extra dollar or so to make it rechargable, then people care a bit less if it eats batteries.

Power consumption on the RP2040 is quite poor, actually; there's much better parts to use if you care about that.

If you just want a really cheap ARM microcontroller, Puya has some parts like the PY32F002A in the <$0.10 range. Single core, lower Fmax, and fewer peripherals than the RP2040, but the price is hard to argue with.