Change Add to Cart on hook form alter when price is zero

file.php

<?php
/**
 * Implements hook_form_alter()
 */
function MODULE_form_alter(&$form, &$form_state, $form_id) {
  // Avoid bundle cart forms and other forms
  if (strpos($form_id, 'commerce_cart_add_to_cart_form_') === 0) {
    // Use the price calculation system
    $price = commerce_product_calculate_sell_price($form_state['default_product']);
    // Some products don't have a price, like bundles. In this case we get "NULL".
    if ($price !== NULL) {
      // if price is exactly zero
      // Note that the amount is in minor units, $1 == 100 and returned as a string, thus, intval()
      if (intval($price['amount']) === 0) {
        // replace button with text
        unset($form['submit']);
        $form['submit']['#markup'] = '<p class="call-for-pricing">Call for pricing</p>';
      }
    }
  }
}