Starting from a post on Sharepoint 2010 Forum, here are steps "How to"
First, i created a new page, in which in first side i inserted the values from a list as they are. One of these columns is a calculated column, and is named: Difference
In the bottom side, already exist the DataFormWebpart modified. The page can be found here.
After I created the page with an initial DFWP I have selected from ribbon "Sort & Group".
In the new window select one single line column, a mandatory column; in my case "Title"
Next I need to group all items, in one group. For this, I will select all items with Title field not empty. To do it, select in the right side button "Edit Sort Expression...". In the new window i will insert a new code, as bellow:
Next step , check "Show group footer":
and select "Show column totals per group"
Click OK button, and now we have in our page sum calculated values for each column. What i have to do is to customize itNow, let's customize the page. First, I will eliminate row named: "Count" as bellow:
For this, select word "Count" and then goto code side and remove the code:
As you can see , the row isn't there anymore.
Last step, customize the row which contain sum values: