Oak Tree Inns $8-10/night in 2018
According to extreme hotel deals, Oak Tree Inns can be booked on major OTAs for $10/night for many dates in 2018
I successfully reserved 6 nights in Dunsmuir, CA for $8/night plus a couple bucks in taxes in July 2018 on booking.com
I was also able to find some dates for Green River, WY at the same price but didn't book them because I couldn't get the later summer dates I wanted at that price.
These are not fancy hotels but they're well reviewed and do offer free breakfast.