Getting value in one column based on other two columns for each row in table

Hello.

I have a table where I have my products, price for each one and quantity. I would like to get total cost for each product based on its qunatity and price. So, Total cost = quantity * price per unit. I saw a rule, Calcualte total cost on this article: http://help.bizagi.com/bpm-suite/en/index.html?performing_actions_on_collecti.htm , but I need to see full implementation of that rule, beacuse I cant find it on any other article.. How its made basically.

Thank you in advance.