Display text of price (minus 20%) on every product page in a sentence.

Assuming you would put this in a woocommerce template file, wherever you want it to display:

<?php

    global $product;
    $regular_price = esc_attr( $product->get_display_price() );
    $discount = 20;
    $currency = esc_attr( get_woocommerce_currency() );
    $new_price = ($regular_price * $discount) / 100;

    echo 'Get this product for '.$new_price.$currency.' with (Promo code)';

?>

Untested, but should do the trick.