We have a small data center here in Brazil to host our servers.
Now we are planing to provide some colocation and VPC hosting, and we need to measure monthly traffic of each ip/port.
We can't to that at router level, since it's rented from link provider and they don't give us access to it.
You say that your "router" is rented from your "link provider", and they don't give you access to it. Does that mean that you have no border or core devices that you control? If so, I'd say you're screwed (and seriously abrogating your responsibilities as a provider of network services).
Assuming you have got something in place that you control, you can monitor in any number of ways.
At the IP level, you'll either want something that exports netflow data from your border to an analysis station. Since you don't give any useful information as to what your environment consists of, there's no specific recommendations I can give, but Google "<your platform> netflow" to get useful results.
At the port level, SNMP statistics should be sufficient to give you billable bandwidth numbers. Again, without knowing your platform, it's impossible to know which SNMP manager to recommend.
snmp at base level, you can look at monitoring tools like cacti, opennms and numerous others and customize the data stored etc.
If you want some very basic information, per port on a managed switch, router, or server, you should use an SNMP or agent based monitoring tool. Nagios, Cacti, Zenoss is my favorite. Now, if you want DETAILED information you should ideally use something like Netflow to and a netflow analyzer to give you some seriously good statistics.
