'Settings', 'General', 'Usage' will tell you the net data sent and received over the cellular network. If you have XSysInfo that will tell you too. However, you need to be aware that this is the net usage, not the gross that includes all the IP protocol overheads. It's difficult to be precise, because this overhead is a fixed amount per packet and the net data in each packet will vary, but add about 25% to the net and you shouldn't be far off. Gross is important because that's what the network service provider will charge you for....
Tim