Apple iTunes has the Free iOS App - DataMan Next: Track Data Usage In Real-Time for Free (normally: $0.99) for a Limited Time!

This app lets you track your Data Usage on 3G/4G network and Wi-Fi. The app creates a forecast of your usage and will warn you if you have a chance of going over your Data Cap for the month.

Requirements

• Compatible with iPhone, iPod touch, and iPad
• Requires iOS 6.0 or later
• This app is optimized for iPhone 5