You could do the calculation yourself:
During the first semester:
You conduct N courses.
Average amount of course material per course V MB
Per course there are M students on average
Average amount of data a student supposed to upload W MB
At the end of the semester your course material will take up N(V + M*W) MB.
Additionally there are the Moodle PHP files, thay don't take up much space.
Then there are the MySQL-data. Can somebody give an estimation?
Logfiles, tmpfiles they should be cleared periodically. Have no figures either.
Continue the calculation for the 2nd semester. Think how many courses will be deleted, how many kept. and so on.
Anyway, always multiply the results by 20, at todays HD prices there's simply no reason to count pennys hier.