According to UK Mouser cheapest ST MCU can run NETMF is STM32F103RET6. I'm currently playing with this one.
But it's better to take STM32F205RET6, since F2/F4 series are widely supported by community.
What is electronic manufacturer you think is going to do for you? If you want complete custom board prototype (design, schematics, board, assembly) get ready to pay 5k+ USD. But if you already have complete project ready to manufacture, the cheapest way it to post buying request on Alibaba.com.
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