//od Pavel Kočárek
add_filter('woocommerce_order_button_html', 'order_button_with_price_html');
 
function order_button_with_price_html($html) {
     
    $custom_text = esc_html__( 'Koupit za', 'woocommerce') . ' ' . WC()->cart->get_total();
 
        $html = '<button type="submit" class="button alt' . esc_attr(wc_wp_theme_get_element_class_name('button') ? ' ' . wc_wp_theme_get_element_class_name('button') : '') . '" name="woocommerce_checkout_place_order" id="place_order" value="' . esc_attr(strip_tags(str_replace(['<br>', '<br/>', '<br />'], ' ', $custom_text))) . '">' . $custom_text . '</button>';
 
    return $html;
}