It might take a little more effort, but if you use the sum tool (Σ) in a plain Excel spreadsheet you can do it for free. If you don't have Microsoft Office, there is a free version of Word and Excel as a "Starter" program. You have some ads to the side, but it's not bad.
I'm not sure if you already know how build a spreadsheet or not, but just in case...
Simply type in your categories, such as flowers, venue, DJ/Band, centerpieces, etc. in one column. In the column next to it, type in the cooresponding budgeted amount. At the bottom of the budgeted amounts column, click on the first blank cell underneath the numbers and click the sum tool (Σ) and it should automatically highlight all of the numerical values above it. If it doesn't, you can click and drag to highlight the cells you want added together. Then hit the "enter" or "return" key on your keyboard and it should give you a total that will automatically adjust if you change one of your budgeted values. You can also use this tool to figure out the subtotals of group categories, such as "decorations" if you would like.
I hope this helped!