There are two options for tracking your bandwidth usage on the iPad. If you tap View Your Account under Settings > Cellular Data, you can see how much data you've used and how much you have left for your billing period. However, I found that this meter doesn't usually update immediately. You can also view usage under Settings > General > Usage, which seems to present a more current picture of your bandwidth situation.
You can check your bandwidth use under the Cellular Data section of Settings, but the count isn't always accurate.
What I learned
When it comes to news feeds, I tested NewsRack with both my custom settings and without. When I turned on some features like downloading images from the latest 10 articles, my first refresh in the morning of my 349 feeds took 15MB. After reinstalling NewsRack to clear out all feeds and use the default settings (with images turned off), an initial download of the 100 most recent articles in each feed took 8MB. Subsequent refreshes to update these feeds later in the day took 5MB with images, and 2MB without.
An example of one of the bandwidth warnings you'll receive on the iPad's 250MB data plan.
Toss in a couple of short YouTube videos and small app updates, and I hit the 250MB ceiling in just three days after activating my month-to-month plan.