Show a running total overall of how much I paid for my collection

I’m adding in how much I paid for a game (some entries going back 30+years ago when I originally purchased them). I see the ‘how much I paid’ column and how much the game is worth now so I can compare that single item.

I’d also like to see the ‘How much I paid’ column automatically be added up to give a total overall of how much I paid, so I can see and compare that with how much my collection is worth now.