Limit downloads on shared ADSL

I have a client that has ADSL, it has a 3Gb limit and then a charge of 17.5 cents per Mb.
Problem is his house is networked and all 4 kids share the connection. The first month they downloaded 22Gb at a cost of $2500, what he wants to do is limit each win98 system to say 500Mb download each per month. Any suggestion would be helpful