Yeah I've thought about powerline adapters but this is an old house. The upstairs still has tube and knob wiring; I'm skeptical that it will play nicely with powerline adapters, lol.
The real problem is that the main feed into the house is in the family room which has been added to the house. The interior wall of the family room is brick and it's making the wifi feed worthless. Maybe the right solution is to drill a hole through the brick interior wall and run an ethernet cable to a wifi repeater. That way the interior of the house should get a decent signal.
But the coax to ethernet adapters are supposed to work well and the price is right,
Amazon.com: Actiontec Ethernet to Coax Adapter Kit for Homes with Cable TV Service (ECB2500CK01): Computers & Accessories