I've been reading about these "3¢ MCUs" and thought I'd publish the links I have.

The "terrible" 3 cent MCU. Padauk's products are on this list.

An in-depth look at the Padauk line and the Hackaday writeup with links to example projects. The 3¢ MCU is a SOP8 OTP part with offline programming, the MTP parts are several times more. So you would normally simulate it first. And if you make a mistake, well it's only 3¢.

An early exploratory project and the Hackaday writeup.

The Padauk product page. The OTP PMS150C is the low end of the line and the one most discussed. You can also drill down to find the Padauk IDE download (quite lightweight) for Windows (even XP) and even runs under Wine on Linux. This interfaces to Padauk's ICE and programmer, for which the open toolchain project below is trying to create substitutes.

The epic EEVblog thread discussing reverse engineering the MCU and creating a free programmer and toolchain. An interesting post there about debugging options and the custom code that the Padauk IDE inserts into every MCU for tuning that chip, important for accurate timing.

The Github Free PDK "organisation" with many projects: software tools, open programmer hardware and software, MCU instruction set documentation, code examples, SDCC code examples, and more. As of Feb 2020, I don't know of any ready made programmer for sale yet.

Small Device C Compiler from 3.9 onwards has support for the Padauk MCUs.

You won't find Padauk's products on eBay. You can buy them from LCSC (with minimum order surcharge and shipping costs so you'd have to make a large purchase to make it worthwhile), and here's a deep link to Padauk products; or some Aliexpress vendors with considerable markup.