Simple traffic counter script or plasmoid

Hi all.
I have subscription to 3G (HSDPA) internet limited by 5GB per month.
I need script or program that count traffic passed through NIC
for control usage.
Network graph plasmoid have such functionality but the problem is that counter resets when reboot.
I googled for solution but not succesfull.
maybe someone must write such software. (i am not programmer) :frowning:

Something like this: