Motherboard for mining is now sold at a low price in Japan. I got it with 2980 JPY (plus tax and shipping cost) at Tsukumo web shop https://shop.tsukumo.co.jp/goods/0889349842522/

It has many PCIe slot, but I have no idea to use them... fill with RS-232C or USB, otherwise Ethernet?

(related article: https://pc.watch.impress.co.jp/docs/news/todays_sales/1160206.html )