Function filters the price of products looking for "Free!" and replaces it with "$0.00"

lets_replace_free_with_zeros.php

<?php // copy this line only if needed, usually it's not needed
/**
 * Function filters the price of products looking for "Free!" and replaces it with "$0.00"
 *
 * @param   string 	$price  	The price being passed
 * @param   object 	$product 	The product we're looking at
 * @return  string	 		The filtered price
 */
function lets_replace_free_with_zeros( $price, $product ) {

	$price = str_ireplace( 'Free!', '$0.00', $price );

	return $price;
}
add_filter( 'woocommerce_get_price_html', 'lets_replace_free_with_zeros', 100, 2 );