Do you know USB-UART converters called CH34x? The type of converter is used in a lot of Arduino clones of development boards.
The same manufacturer designs own microcontrollers based on C51, that's core which was used in a lot of Atmel microcontrolers like AT89C2051, AT89C51 and more.

If you think the core is old and useless, that's wrong idea. Modified core is used in a lot of microcontrollers - for common using like CH55x and also for special application e.g. IC for watches.
There are a few advantages - you can modified the core as you want and is not neccessary to pay for licensing.

The list of microcontroller is below:


Source: http://www.electrodragon.com/w/WCH#MCU_.2B_USB_CH55x_Series

The cheapest board contains CH552 (16 kB ROM, 1kB xRAM, 256iRAM, @24 MHz) which is available in packages which are easy to solder - TSSOP and MSOP.
The price is 1.54 USD.
The more powerfull microcontroller, CH554, is available on the same website and costs 1.78 USD.



Source: http://www.electrodragon.com/product/ch552-ch554-mini-dev-board-ch55x-series/

If you want smaller board for 1.80 USD, check the simplest board with CH551.



Source: http://www.electrodragon.com/product/ch551-mini-dev-board-ch55x-series/

The "most expensive" development board contains the most powerfull microcontroller - CH559. It contains 60 kB FLASH for code, 1kB DataFLASH, 3kB for Bootloader, 6kB xRAM and 256b iRAM. The maximal frequency is 24 MHz and the IC contains USB 2.0 interface.





Source: http://www.electrodragon.com/product/ch559-mini-dev-board-ch55x-series/

More info you find on this github repository - https://github.com/Edragon/WCH