You'll need to include more than a basic level of development and calculation in your spreadsheet. There are lots of free spreadsheets on the web. Most of which are very simple and easy to build. People aren't going to buy a spreadsheet that they can get for free. Take yours to the next level by adding all the value you can imagine. Then research on how you can offer a little bit more on the development and functionality side.
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.
You may also want to track your current spending on one sheet with columns for different categories. Some individuals will list the budgeted amount in one column and then list the actual amount spent in the column next to it.
Protection of formulae : How often have you forgotten to protect the cell contains the formula that took you hours to test and perfect. It takes one instance of honest human error, and that formula is gone.
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