Some routers have statistics monitoring of number of packets and number of bytes transmitted and received. My Netgear RP114 does. Just check it once in a while, and if you get close, tell your employees to back off a bit on the usage. Just reset it once the month is over.
umheint0's phat setup --> <A HREF="http://home.cc.umanitoba.ca/~umheint0/system.html" target="_new">http://home.cc.umanitoba.ca/