FriendlyElec’s $75, RK3399-based “SOM-RK3399” COM/SBC hybrid can stand alone or expand with a $120 “SOM-RK3399 Dev Kit” with -20 to 70℃ support and M.2 and mini-PCIe expansion.

Source : Hybrid RK3399 COM/SBC hacker board can plug into feature-rich carrier