It's almost impossible to give an 'average' cost for a working edition of Moodle without knowing what sort of scale you are thinking of
My Moodle sites run from:
localhost - sits on my laptop/desktop, installed and managed personally, cost £0
home webserver (web accessible) - installed and managed personally, cost £0 - unless you count my broadband, but I would have that anyway
shared hosting - hosted, but Moodle managed by me, has previously been used to support a primary school moodle instance, cost £7pm
small external site for a university - partner hosted, cost unsure but in the low 000's
main university moodle site - internally hosted, moodle managed by me, server managed by IT staff: server capacity and staffing already exists - but by the time the various staff also do other tasks and work with other systems if you took one full time annual salary you'd probably be in the ball park of the specific cost (but this wouldn't cover a bigger institution like OU, so you still have scale to think about there)
In terms of personal education and experience - I started running a localhost and school website (probably about 10-12 years ago) with no experience and knowledge at all. Moodle has been my personal/professional development vehicle for learning about linux server management and web development. That's fine for running a small personal site, and may even be OK (given time commitment) for running a small school site, but running a university site requires more expertise in the server management, security, backups etc. BUT in terms of comparative costs between Moodle and AN.Other system, bear in mind that those same costs would apply (along with Tim's comments around content creation) to any self-hosted system.