We have some 1400 users and about 140 courses.
We use 3.4 GB in the moodle data folder and an additional 10.8 GB in the moodle backup folder.
If you back up to the course directories, you must add the two
together. It depends on how many backup files you keep, because
as admin you can set up a system that suits your needs.
I agree with Anthony, this depends a great deal on what types of
resources your instructors use. Multimedia files can take up quite a lot of space!
All files that are uploaded to the course are kept in the moodle data
folder, not the database. Even files uploaded by students.
(there is a folder in your course files called moddata. The
student files appear there). I believe that the database only
contains links to the files, not the files themselves. It is my
understanding that the database does contain text, such as the messages
or the web resources. That is why those are very limited in
size. (256K?) I can't quite remember!