Actual data usage on the carrier side may surprise you. I've always thought of myself as a semi-heavy user (at least for data). I have 3 exchange/push accounts (work and personal) running daily for email, contacts and calendar. I also surf at least an hour a day, most days closer to two. I use Google Maps daily (it supposedly uses quite a bit of data). In addition to that, I view at least a few news and YouTube videos a day. On occasion I also tether my iPhone to my wifi-iPad when away from home.
FWIW, I have wifi at home but my work wifi is very spotty. For my usage I figured I should be on a 6GB data plan which I've been on for 2 years. Just last week I requested a break-down of data usage from my carrier, and I was shocked when I saw my actual usage.
It turns out I've been averaging 200-300 MB/month. Only during two month in the last year I've gone over 500MB (680MB was my highest month).
Needless to say I dumped my 6GB plan for 2GB which is saving me a few bucks. I think it's best to trial run with a larger amount than what you anticipate, and then after a few months request your usage from the carrier (don't rely on your iPhone meter - they usually are quite a bit off from each other) and only then you'd be able to really tailor a data plan for your usage.
My own formula for data plan is based on double what I've been averaging plus a bit of buffer (which in my case is ~2GB).