Add an entity property for the Node which can be used as Search API index field

/**
 * Implements hook_entity_property_info_alter().
 *
 * Adds an entity property for the node.
 */
function HOOK_entity_property_info_alter(&$info) {
  $properties = &$info['node']['properties'];
  $properties['custom_price_calculated'] = array(
    'label' => t('Custom calculated price'),
    'description' => t('The Price after applying all the changing components.'),
    'type' => 'decimal', // options: integer, boolean, ....
    'sanitized' => TRUE,
    'getter callback' => '_get_my_component_price',
  );
}

/**
 * Get the amendments price by all applicable components.
 */
function _get_my_component_price($data, array $options, $name, $type, $info) {
  $discount = _get_discount_info($data); // custom function.

  if (isset($discount['discounted_price']['amount'])) {
    $price = $discount['discounted_price']['amount'];
  }
  else {
    $price = NULL;
  }

  // Convert the value to decimal value.
  $currency_code = isset($discount['currency_code']) ? $discount['currency_code'] : 'EUR';
  $amount = commerce_currency_amount_to_decimal($price, $currency_code);

  return $amount;
}