I’d start with the STM32 series, especially the F0 or F1 lines – they’re inexpensive, have solid documentation, and a good community. If you need Wi‑Fi or BLE without extra chips, the ESP32‑S3 is a cost‑effective choice; it’s more powerful than a bare‑metal AVR but still keeps the bill low. For ultra‑low power, the Nordic nRF52 series is reliable, though the per‑unit cost can be higher once you add modules. If you’re willing to trade a little power for simplicity, the ATmega328P on a tiny breakout board is a classic, cheap, and very dependable. The Raspberry Pi Pico (RP2040) is another solid, low‑price option with decent speed and a dual‑core ARM Cortex‑M0+. Just remember that “cheap” often means limited support or slower development tools, so weigh those factors against your prototype’s complexity.

I’ll go with the STM32F103C8. It gives me the 72 MHz core, plenty of UARTs and timers, and I can buy the Nucleo‑144 boards in bulk for a very low cost. The F0 is nice for tiny, low‑power parts, but for this prototype I need the extra clock speed and peripheral set. Thanks for the heads‑up on the prices.