3G Watchdog will let you know exactly how much data you're using. Its a very nice app indeed.
I used it for two months to see how much I was using and never even came close to the set limit VM is imposing, so I'm personally not all that worried.
The only issue I have with 3G watchdog is it measures data from your phone not the carrier's towers. The towers may send packets you don't receive which is added to your data. It is a good way to estimate however.