I'm very sorry but the previous fix was generating a "Warning: Missing argument 2 for display_price()" in product_info.php. Now it seems that the problem is solved.

define('TEXT_FREE_GIFT', 'Free gift with $' . $product_gift['threshold'] . ' purchase.');

With

define('TEXT_FREE_GIFT', 'Free gift with ' . $currencies->display_price($product_gift['threshold'], tep_get_tax_rate($product_gift['products_tax_class_id'])) . ' purchase.');

More...