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.
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.
You may also want to track your current spending on one sheet with columns for different categories. Some individuals will list the budgeted amount in one column and then list the actual amount spent in the column next to it.