I was just browsing itunes, and I found an app called "Bandwidth" (without the quotes
).
It seems pretty good, you can click on "plan" in the top right corner of the app, and type in your max bandwidth for the month, say 250mb. And click the "date" in the top left of the app, and choose the date the monthly contract starts. Then there will be a status bar, showing how much bandwidth you have used in MB and in %. (you can have it display in Bytes, KB, MB, and GB)
Under that, it shows how many days into the month's contract you are, and how much you have downloaded, and uploaded.
Theres a pro version of the app, called Bandwidth Pro (how original lol) which costs £1.19, but the free version seems to do exactly the same thing. (there is an add at the bottom of the free app. Thats probably the only difference lol)
Anyway, heres a pic of it from my ipod (It's an iphone app, but who cares if it doesn't look pretty on the ipad, long as it works lol). I updated 1 of my apps to see if it worked, if you are wondering why it already shows 25mb.