Tip courtesy of geeksugar.com.
First, head to your iPhone's settings app. Scroll down to "General," then click on "usage." You'll see your minute breakdown, but at the very bottom you'll see your data usage (sending and receiving) over the cellular network. If you've never reset your stats, you'll find that this number will be fairly high.
You can reset your statistics today (by hitting the "Reset Statistics" button at the bottom of the screen), and check back next week to estimate just how much bandwidth you're eating up each month, and determine if you want to switch to a less expensive plan.
I use a free app called DataUsage.
I won't go into the details as its pretty obvious what it does, but it is perfect for what you want. Available on the AppStore.