$20 · PowerLine AV 500 Starter Kit

Also called the "DHP 309 AV" 500 Mbps Powerline adapter; it runs Ethernet across the power lines inside your house. Two units included but I think you can get more? Each adapter requires one power socket and accepts one Ethernet cable. Performance is pretty good when the units are pretty close to each other and nothing is generating too much noise, i.e. plugged in with lamps, fans, or something like that. If you plug it into the same socket as your $10 hairdryer you're bound to have a bad time. More details: https://www.dlink.com/en/products/dhp-309av-powerline-av-500-mini-starter-kit