How to do some calculations using fields within a Field Collection field?
You can do it by using Computed Field.
The sentence 
$sum = 0;
$wrapper = entity_metadata_wrapper($entity_type, $entity);
foreach($wrapper->field_FIELDCOLLECTION as $collection) {
$sum += $collection->field_SUBFIELD->value();
}
$entity_field[0]['value'] = $sum;