TP-Link TL-WR700N Wifi Router for $10 with free shipping.
Normally $20 to $30.
Range, features, and Wifi load will be limited on a cheap router like this.
It may be best when used as an "access point" or "bridge."
To use an an "access point", run an Ethernet cable from your main router (or Switch) to somewhere in your home that has a weak Wifi signal, plug it into this device, configure as in AP mode, disable DHCP, and set up WPA2 Wifi encryption.