You’ll need a plugin like RepeatingGroup Tools by BDK to access the values inside the RG: RepeatingGroup Tools (bdk) Plugin | Bubble
You drop an element inside the RG, tell it what fields to reference, and what ‘type’ they are. You then drop a parent element inside the page outside of the RG, and tell it what to do with each data type. In your case you would most likely use a :SUM operator on the outside element to sum up the calculations inside the RG.