I have just cooked up SushiBits M23 Mini Pro, an ATSAML11D16A board with a STM32F042F6P6 as an onboard CMSIS-DAP debugger. It should be available at $9.99 + shipping on Tindie once I get the built-in debugger firmware ready. It has the same physical size of a DIP-24 0.6in chip - probably the smallest ARM development kit with built-in debugger ever.
I added it to the list. I think it could be cool if you could expose both the STM and SAML SWD pins (Maybe this is already the case).
It is already the case. However I don't think I can make it within US$10 right now due to what few vendors that carries SAM L11 jacking up prices.
If you step up to the sub-$30 bracket I have a lot more offerings, all of which Arduino capable:
* SushiBits One Pro v6 (designed for STM32F103/F303/L152, but also supports Chinese chips like GD32F103/F303, AT32F403 etc, built-in debugger based on DAP42)
* SushiBits ARM Classic Pro v2 (LPC2103 with DAP42 onboard debug, ARM7TDMI core)
Take another step up to sub-$50, you get Linux boards in form factors other than Arduino
* SushiBits Media 64 (Allwinner V3s with 64MB built-in DDR2, 64MB SPI Flash)
* SushiBits Media 512 (Allwinner A13 with 512MB DDR3 and 16GB or 32GB eMMC)