Average according to the table visual

Hi Fam,

I would like to get the average of the spending of all customers in the table visual below. The average spend column in the visual is a calculated column in the customers table where everything on the visual is pulled up from. Furthermore, I have buttons where I filter the table which is from a different table ( items table ) that has a one-to-many relationship with the customers table. 

Actual goal: 

A card visual that shows the average spending of all customers, just like the average_item_count below.

 Please help.