Can I use JotForm to fill out a form that adds up the numbers in different fields like Excel?

Hi there

I would like to prepare a form for calculating the cost of an event. I can send this to people who need to run an event, and they can input their projected costs, and I would like the form to calculate the running totals, similar to an Excel spreadsheet. 

Is this possible to do using JotForm?

Yes, it is possible to calculate field values here in JotForm. You can use the Form Calculation widget and Update/Calculate Field condition.

I recommend you to check these guides:
How-to-Perform-Form-Calculation-Using-a-Widget
How-to-Insert-Text-or-Calculation-into-a-Field-Using-Conditional-Logic
How-to-Assign-Calculation-Value
How-to-Perform-Form-Calculation-in-the-Matrix-Field

After that, you can pass the total value to the payment field. Here's how: How-to-Pass-a-Calculation-to-a-Payment-Field.

Hope that helps. Thank you.