The Rates window gives you a price breakdown for each Resource Rate you have added to the Buildup of a specific item.
The Item Value column displays the specific Resource Rates value for the whole Item (whether you are using single Buildups or Composite Buildups).
The Element Value column is used when you have Composite Items as you will have multiple single Buildups against the same Item and each selected single Buildup Rates will be displayed in the Element Value section.
Note: The Element Value column can be activated by right clicking the top toolbar in the Rate Window and selecting columns.