How to Sum up values from a specific field, only if another field value is the same.

I have the three fields:

Receipt # Group FIELD

Receipt Items FIELD

formula for Receipt Items right now:

let myNum := 'Receipt # Group';
cnt(select Inventory where myNum = 'Receipt # Group')

Purhcase Cost FIELD (dollar amount)

I would like the sum of the Purchase Cost of all specific fields ONLY if they have the same Receipt # Group value.   For example, out of 500 entries, I have, let's say 27 of the same Receipt # Group value.  Those 27 entries need to be added up.