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.
If you have Microsoft Excel, you can create budgeting spreadsheets to help you manage your money. Instead of buying expensive software that will only be used for your budget, spreadsheets can be used for a variety of tasks. If you don't have Excel, you can find free spreadsheet applications online in which to manage your budget.
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.
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.