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.
When your spreadsheet start to hit its limits, what options are open to you? Perhaps the best option is to consider the conversion of your spreadsheet to a database solution. Relational databases became viable not too long after the spreadsheet was well-entrenched as the affordable and quick means of gathering data and performing calculations.
Rather than having to go through the pain and suffering of learning a new technology and having to experience trial and error development, the best route would be to acquire the services of a consultant, who can identify your needs and create a customised solution. Obviously, the cost of such a project would depend on the size and complexity of the spreadsheet and your requirements. However, you might be surprised at how cost-effective this project would be.
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.