In 4.12 we have introduced showback features. This toolset generates cost reports that can be integrated with chargeback and billing platforms.
Now each VM Template can optionally define a cost, in undefined units. The cost is defined as cost per cpu per hour, and cost per memory MB per hour. There is also a default cost that can be applied to all VM Templates.
OpenNebula will then calculate monthly reports that are nicely presented to both the administrators and the end users.
And, of course, the raw data can also be retrieved in xml, json or csv from the command line. For more information, go to the 4.12 development documentation.