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.
Once you design the type of budgeting spreadsheets you want to use, it will be easy to copy it for subsequent months or years. You can also use just one sheet and create tabs for each month.
You'll need to develop good marketing material. Things like demo videos, which you can find plenty examples on YouTube, screenshots, and keyword rich descriptive text help people find your spreadsheet and communicate the value that sets yours a level above the rest. Excelville.com actually allows you to upload your spreadsheet with all of this marketing material in an easily searchable and user friendly website.
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.
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