You might look into running a replacement firmware on one of the many
routers that will take it. DD-WRT is the most versatile and the newer
versions have bandwidth monitoring, I believe. Certainly their
paid version does and in any case, DD_WRT has Rflow, which will help
you monitor usage.

DD-WRT.com has a long list of compatible routers that can be flashed
with one version or another (be careful to get the right one).

If you go that route, the favored Buffalo routers, which are currently
not sold in the US, are available, preloaded with DD-WRT from their
store.

Also, many are very pleased with "Tomato a" simpler firmware
replacement that concentrates on BW monitoring. http://www.polarcloud.com/tomato

If you don't want to have a buffalo shipped from Germany, then the
Linksys WRT54G series works with DD-WRT (it's namesake). Be sure and
check the version you are buying against their support list. The
WRT54GL (note the "L") is a safe bet for either firmware.

Steve