I'm very happy using Mikrotik (mikrotik.com) products. You can buy a ~49$ license and install their software -RouterOS, a Linux derivative- on an old PC.
You can also get one of their hardware products (routerboard.com), excellent choice if you care about power consumption. I highly recommend the RB750G wich is a small SOHO router in a nice plastic case.
With RouterOS you can do a whole lot of interesting things, monitoring bandwidth usage is just one of these things.