How to calculate the sum of 2 fields in a content type and display the result in a 3rd one when the node is saved?
Computed Field is a good solution. You can achieve a similar results without an additional module (provided you already have a custom module).

If you need to store the result when the node is saved, you can implement hook_node_presave() to set the value of the third fields just before the node data is saved to the database.

MODULE_node_presave($node) {
  $value = 0;
  $price_items = field_get_items('node', $node, 'field_price');
  if ($price_items) {
    $value += $price_items[0]['value'];
  }
  $vat_items = field_get_tems('node', $node, 'field_vat');
  if ($vat_items) {
    $value += $vat_items[0]['value'];
  }
  $node-&gt;field_value[LANGUAGE_NONE] = array(array('value' =&gt; $value));
}


Because the value is always calculated and simple, there is really no need to store it in the database. If your are already using the Entity API module and its entity metadata wrappers, this is easily doable with a hook_entity_property_info() implementation.