If you want an easy way to control how much you spend on your phone, My Data Manager is an iPhone application that can help you out.
Save on your phone bill.
Every day you'll be able to keep an eye on the data you've consumed on your smartphone via data connection, Wi-Fi or roaming.
Keep control of your data consumption
One of the main advantages of My Data Manager is that not only can it be used to monitor the use of your data (both current data and monthly data), but it also lets you set alarms that help you not to go over the top on your consumption.
Download My Data Manager free for iPhone if you want to have full control over your phone bill.