How to create expression on save of collection

I need to calculate total cost * quantity in expression of everytime user add new item in list on same form

I am following this tutorial http://help.bizagi.com/bpmsuite/en/index.html?store_the_total_value_of_a_col.htm

but I don't know how to set expression on save button for collection, also I need to change currency from $ to £.

Many Thanks