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.
"Spreadsheets for Small Business" is a complete package of ready-made spreadsheets that optimizes and facilitates a series of business processes such as sales exec commission tracking, calculating business expenses and planning budgets. The "Spreadsheet for Small Business" package incorporates valuable programmed spreadsheets such as "Small Business Expense Spreadsheet" - provides accountants and tax-preparers with a complete report on relevant business expenses; "Producer's Budget" - helps users to establish budgets and keep track of their spending; "Monthly Personal Budget" - designed to track daily, weekly and monthly spending for individuals.
You could make money doing something you love (or like [or maybe not so much but just really good at]). It's great to see a spreadsheet come together, and usually a lot better than what you originally envisioned. Especially when the dang thing works.
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.