That's a big ask. Right now RISC-V has a handful of open source CPU cores. There's no open source network, graphics, peripheral and GPIO hardware that can be cheaply plugged into a synthesized design that can be sold at the price you want. Which is to say, while there's plenty of stuff out there that someone made work on a FPGA once, there's nothing with a track record and validation/driver stack ready that you can throw into your tools and expect to get a working chip with. And that's what those SoCs you find in the Pi and similar boards need if you want to get them out at the small-integer-dollars price point: no-brainer existing IP that won't surprise anyone along the manufacturing and support chain.