How to show dynamic text from a page state on the fly

I can't display the result of a function on the screen.
I have a database with items, and these items have prices.
I have a list view to display the shopping list and another list view to display the items already in the cart.
I have a toggle (purchased = true/false) to move items from one list to the other.

I want to display the total value of the items in the cart (purchased = true). I wrote a function that sums the items that pass through the filter and saves them in a Local State Page Variable.