A typical PC might have an idle power consumption of less than 50 W, while a very small computer with ARM little cores might have an idle power of 2 ... 3 W, so at most the power consumption ratio would be 20.
However even that is not realistic because a computer or appliance with less than 3 W power consumption will not be able to route 10 Gb/s or faster links and it will struggle even with multiple 1 Gb/s ports.
A dedicated router appliance able to route 10 Gb/s or faster links will probably have an idle power of around 10 W or even more.
For routing only 1 Gb/s links, you can use among the standard PC's a NUC-like model, which will have an idle power consumption between 5 W and 10 W, quite close to a dedicated appliance. This is actually what I am using for my own Internet router/firewall (which also runs many other services, e.g. DNS server and proxy, NTP, e-mail, HTTP server and proxy etc.) with 1 external 1 Gb/s port and 4 internal 1 Gb/s ports (4 of the 5 ports are made with USB to Ethernet adapters).
Some of the more recent NUC or similar computers have multiple Thunderbolt ports or 10 Gb/s USB ports, so those can be used to route multiple 10 Gb/s links (using Ethernet adapters), with an idle power of around 10 W, similar to any equivalent commercial routers.
For 25 Gb/s Ethernet links, a commercial router is unlikely to be much cheaper or to consume much less than a standard PC.