iNetUsage is Cydia iPhone tweak that has a function to check your iPhone traffic usage. Though this app is not free, but you can get a great advantage to monitor your network bandwidth usage.
After you have installed iNetUsage on your jailbroken iPhone simply specify a date range and see your network usage broken down by application and network device. This tweak can be found under the General -> Usage panel in the Settings app.
iNetUsage is available in Cydia under BigBoss repository. It costs $1.99 USD. This app requires iOS 3.2 or higher.