Been looking at low-end chips the past few days.
I find that Freescale is having a go at poaching customers from the AVR/PIC worlds, with the Kinetis E series: runs on 2.7V to 5.5V, is decently appointed with memory and peripherals for competing with the ATmega line... and starts at around $1 in onesies.
Then, today, I go looking at FPGAs for a rather specialized application (for which I can't easily find an MCU that'll do the job, within the given constraints). The Lattice ICE40LP series, which I'd been considering for some bigger applications... starts under $2 in onesies, for a device that won't handle the task at hand but appears useful for other purposes. And the slightly more expensive model that probably would do the trick, clocked at the necessary frequency for my purposes, looks like it'll run on under 5mW.
These things need feeding, of course. So I look up dinky, low-power buck regulators, for getting down from lithium-cell voltage to MCU-and-FPGA voltage (assuming a 3.3V-or-lower MCU). Well! TI has a handy chip that comes in a SOT-23-5 package, runs at 1 MHz, and has a low-power pulse-skipping mode and 15µA quiescent current. And, under a buck in onesies.
Might end up designing the whole mess around lower-voltage logic; maybe 2.5V rather than 3.3. But there's also an analog section, so that means shopping for low-voltage CMOS op amps.
I may just be able to put most of the analog signal chain inside the FPGA, which would be mighty handy. That would leave just some preliminary signal conditioning to be done with an op amp or two.