Get the data first, then display it conditionally. Also avoid changing too often between php and html to get more readable code.
Use the variable expansion in double quotes strings.
$property_price = get_property_price($post->ID);
$rentperiod = get_post_meta($post->ID,'rentperiod',true);
if($price && ($property_price || $rentperiod))
echo "<li>$price: $property_price $rentperiod</li>";