In sales by items in the up right corner are 3 points; if you will click on them will appear the posibility to display more fields in you table. One of them is cost of goods. If you choose to display the cost of goods then you may see the total cost for each item. If you would like to calculate the total cost of all goods then you can export the sales by items report and calculate the total cost of sold items.
Also another way is to download Loyverse Dashboard app and then you can see the total cost of sold items in Sale summary report. https://loyverse.com/dashboard