You can use th tc command, combined with the tbf (tocken bucket filter) to put a cap on the total monthly outgoing data.
If you have a line with 10Gbps, it can go up to 3300 Tb per month. And if you have a limit of 500 Tb per month, the maximum bandwith is
500/3300 Gps = 150Mbps
The command to enter would be :
tc qdisc add dev eth0 root tbf rate 150.0mbit burst 20kb latency 50ms