New: check out the new STM8 eForth MODBUS project on GitHub - the code is compatible with this board :-)

Some vendors on AliExpress give "11.11 Sale" rebates on STM8S103 Relay Boards: the lowest price I've found is at $7.11.

Even if you add the $1.85 for a ST-Link V2 clone, and the $0.60 for a RS232-USB adapter you'll stay below $10 for something that's not only good for experimenting with interactive embedded Forth, but that's also OK for real world control applications!

The C0135 image in the STM8 eForth Binary Release works from out of the box, and the STM8 eForth Wiki on Github has a page dedicated to the C0135 module.