DataMonitor is free jailbreak application, which monitors all processes and data on your iPhone.
This Tweek shows detailed statistics about the use of cellular (2G, 3G) and Wi-Fi traffic.
You can adjust the thresholds of traffic for a day or a month and set the alarm for the exhaustion of 50%, 75%, 100% of the limit.
DataMonitor utility maintains a record of daily and monthly usage data, and provides information about battery, memory, CPU load, information about running processes, system time, uptime, and much more.
This tweak will provide you with pretty much whatever one could ever you want to know about your iOS device.