Replace empty price (0) with a link such as "Request price"

This script will replace “0” (displayed when the price field in a WooCommerce product is left blank) with a custom link.

Add the following code to your theme’s functions.php:

add_filter('woocommerce_empty_price_html', 'custom_call_for_price');
function custom_call_for_price() {
     return '<a target="_blank" href="URL HERE">CAPTION HERE</a>';
}

Source : unknown