Since there isn't an easy way to see what on my list was purchased and what was a freebie, it's kind of difficult. I have 764 items in my Purchased playlist, but at least half (I think more) are freebies, especially since now there are something like 2-3 songs/week and I try to get them all if I can.
Overall though, I think I run an average of $1-2 per week, over the four years that I've had iTunes now. All songs -- I've only purchased whole albums a few times, and I've never purchased video content yet. So the $300-400 or so I've spent is a lot in one sense, but in a rate sense it doesn't bother me too much.
Interestingly, though, I had a pretty big uptick in physical purchases (almost always used) over the same time period... so I probably only do 5-10% of my music spend on iTunes.