There is an app called Download Meter.
Download Meter measures how much data you use via wifi and 3G service. Since I have an AT&T data plan, this app gives me a very good idea of how much data I've used.
Originally Posted by JWKessler
It is hard to gauge how much data service you'll need. I barely use the 250MB us$14.95 plan due to readily available wifi service. You may have noticed for AT&T plans it is easy to get 3G service when you need it. You should also be aware that the AT&T plan auto renews, so make sure you cancel your plan at least a day in advance!