I was working on a similar solution as stefanop (we are far away to have it ready) our setup is a PCIx1 child board that plugged to the motherboard and linked with eSata Express cables to an board using moreless the same chip in 1-3x switches ( PI7C9X2G404SL ), with 3 PCIe x1 ports with open ends so the GPU are plugged directly to the PCIe Swith board 3 GPUs (to keep the board practical for production, each board is about 9" and provides about 1 inch per gpu clearance for cooling, is powered by the eSata Express cable draws no power from the motherboard, and we hope it could be stable enoug. my plan is to plug 6 of these on Biostar Motherboards (assuming we solve the bios issue), each 3 ports board inlcuding cable has a BOM cost about 45$ (not including manufacturing) so out target numbers are close to that from stefano, of course I follow stefano development singe he has solved the gruesome dirty development we just started, in case their boards are good enough I'll not mess with my design and buy/commission from stefano.

I'd like to know about stefano approach to solve the link issue (custom flat ribbon cable, eSata Express, U.2 even multiple USB-C are valid as long has enough lines to full link the PCIe port (18-20 lines total, but it could be less w/o redundant lines, but not so few as with USB3 10 lines).