Hi Marculos and thanks for posting!
To extract data from a repeating section, you will need to map it to a list. A standard ShP 2010/2013 workflow cannot extract repeating data from an InfoPath form.
There are two ways to map the repeating data to a list. You can use CAML or qRules (common code library).
You'll want to do this from the form when it is submitted via rules on a button. Make sure to do this before the final submit. If the list mapping fails, you don't want to submit the form.
Please search for "repeating data CAML" or "repeating data qRules". There are many posts.
To do the calculation, you may need to either a) read it into the form and update it vi a separate list, or b) use a calculated value in the first list (not sure this is possible).
Good luck and let us know if we can help you.
Regards,
Patrick