By default there is no obvious hook for the normal price in the current version of Zen Cart (I hear a whisper that this will change in the next major upgrade).
If you want to add tags onto which you can hook some styling (as has been done on the site referred to earlier in this thread), then you could try this ...
... includes/functions/functions_prices.php, find this line
PHP Code:
$show_normal_price = $currencies->display_price($display_normal_price, zen_get_tax_rate($product_check->fields['products_tax_class_id']));
and replace it with
PHP Code:
$show_normal_price = '<span class="standardprice">' . $currencies->display_price($display_normal_price, zen_get_tax_rate($product_check->fields['products_tax_class_id'])) . '</span>';