[Skunkworks] Wifi billing Software

Hello Skunks, I would like to know if there is a solution out there for billing users in a WiFi / hotspot connection that is not pegged on the duration you stay online but by the packets of data downloaded and uploaded. I am thinking of having a small wireless router at home and charging my neighbors monthly according to the data volume they use. like the way bundles work. Further, has anyone tried it in this list? Any free ware software you would recommend? Thanks, Mutuma