These are the pros and cons of selling spreadsheets. The real beauty in this idea is that you could do the work once and there is no limit to how much you can earn, especially with effective use of the internet and other mass communication tools.
Size limitation : When a spreadsheet is first constructed and tested, the size tends to be small. However, as the spreadsheet grows with more and more data, it start slowing down, until it reaches that point where your feet go to sleep while you waiting for the re-calculation to finish.
You may want to create a workbook with multiple sheets for various aspects of your budget. You can have one sheet that keeps track of savings and investments and another sheet that lists your monthly bills.
Inconsistent data : This becomes most apparent when trying to find a particular record or cell, based on the contents. If a space is incorrectly added in the wrong place, the search or filter fails.