Just for the record, with my current mobile browsing habits with the phone switching to wifi in pretty much all "known locations", my average traffic on 3G is 1,5gb a month or so.
That said, I have unlimited amount of traffic, but the speed is capped to 0,5mbit (we dont really have much connections with limited traffic around here), so I dont really care much to fe. watch videos and I just dont care to stream music either, I just listen to what I've transferred to my phone. So perhaps if I had a data plan with unlimited speed and limited traffic, i might have more traffic used due to watching more videos for example.
I could still squeeze that much lower, I dont use the wifi at university due to not bothering to log in when 3G works fine there. I also travel in a train a total of 3h every weekday and the train has wifi, but it's 3G based connection too and my carrier has better coverage in the area the train goes than the one the train company actually uses, but I could probably squeeze off at least that 500mb (maybe even 1gb..) more by using wifi always when possible.
But anyways, thought it might count for something, so average 1,5gb a month for me.