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.
Financial templates are highly complex and they deal with very large and advanced calculations. These spreadsheets are used in mortgage, loans, and property investments and so on.
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 can build a reputation as an expert spreadsheet developer. This certainly doesn't hurt the resume. Using Excelville.com to sell spreadsheets allows you to set up a profile with your work, biography, and other info that certainly makes for good conversation material during interviews.
what is a spreadsheet used for