I wonder why no one didn't invent it still!
It is very simple. Look on the display of any mobile. What indicators do you see? Correct, connection level and battery level.
But there is at least one thing you have to check time to time. It's your balance - money.
Why not to create separate online indicator for money? Of cource, there are some issues - it will be appropriate for prepaid tariffs only.
There are some details:
1) User can set a maxumum level of such indicator by himself. For example, for me - I usually fill up my balance to $10. It is my maximum. I can set it up in preferences of my mobile.
2) Indicator refreshes after each call, SMS or internet connection. I don't know technical issues very well, but it can be done by trivial USSD-request to operator, I think.
3) Of cource, all request and answers are different, that the puzzle for technical guys: how to make a phone to read balance from operators billing system. Maybe, its time to create come standard guides?
Well, that's my idea. I 'll be glad to read your answers and discuss it!