I mostly only use 3G while I'm at work, for some quick Facebook checks and MacRumors reading. Rest of the time is WiFi.
I have a £25 p/month plan and I don't use anywhere near that much data. I get 15GB a month and use about 2 or 3GB. BUT, I am glad because it may aswell be unlimited to me and when I am out and about I never have to worry about not having internet. It also help that I am with UK's best provider for 3G coverage.