Regarding a balance box: well, it is reasonably easy to implement. Will look into it when I get some spare time.

But I created it as a fast way to register these small expenses, and the balance I check on a spreadsheet (using Excel, you only have to open the TXT file and select the value column to see the total). Actually, I use the N900 just to register the information and from time to time transfer this information to a large worksheet.