Shenzhen Xunlong Software's Orange Pi RK3399 single board computer launched in early 2018 with 2GB RAM for $109, and earlier this year, the company launched an updated version with 4GB RAM and a lower $99.96 price tag.
But there are plenty Rockchip RK3399 SBC's on the market, including FrienglyELEC NanoPi M4 going for $50 and up ($75 with 4GB RAM), and Pine64 RockPro64 board starting at $59.99 with 2GB RAM, and $79.99 with 4GB. So unless you need the extra features (HDMI Input, SATA port, mPCIe socket...) offered by Orange Pi RK3399, other boards may be more competitive.