If you just want a number to quote in a thesis, then looking at what a Moodle Partner would charge for hosting to get a rough figure, is probably the best thing.
However, I suspect that for any real Moodle installation (or any other VLE for that matter), most of the cost is in using the system effectively, not the hardware and installation costs.
For example, you have to give your teachers time and training to learn to use Moodle effectively in their teaching. You have to have support available to help people who have forgotten their passwords, or have other similar problems. You have to do some maintenance to create new courses each year, and archive old ones. You have to run regular backups. You have to keep your installation up-to-date.