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 might be providing an extremely valuable service to many, many people. Your spreadsheet might free up peoples time from boring and tedious administrative work to spend more time doing things they love. It might help keep businesses above water or help them reach for new heights. Your spreadsheet might even help to create jobs or at least keep people employed.
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.
One user at a time : In the instance where there are a number of people required to work on a spreadsheet, they can only access it when no-one else is working on it.