Affect single Product price without affecting related products beneath

Question

The thing is that I wanted to add a hook on single page, so that if the product hasn't got a price then add a custom button that leads to contact page. The validation I make on the hook is that if I am on single product page. Everythings works fine but the thing is that I also see the button on related products beneath. How I can hook only inside the current single product that I am looking and not the single product page? This is the code I use now:

add_filter('woocommerce_empty_price_html', 'custom_free_price_text');

function custom_free_price_text($product)
{
   if (is_product()) {
       global $product;
       $price = $product->get_price();
        if ($price == '') {
            ob_start();
           //my code..
        }
    }
}