Hi,
You would need to edit the file "option_price" via the menu Display->Views and change the line:
echo '<span class="hikashop_option_price_title">'.JText::_('PRICE_WITH_OPTIONS').':</span> <span class="hikashop_option_price_value">'.$currency->format($price, hikashop_getCurrency()).'</span>';
to:
echo '<span class="hikashop_option_price_title">'.JText::_('PRICE_WITH_OPTIONS').':</span> <span class="hikashop_option_price_value">'.$currency->format($price, hikashop_getCurrency()).' ('.$currency->format($currency->getUntaxedPrice($price,hikashop_getZone(),XXX,2), hikashop_getCurrency()).')</span>';
where XXX is the id of the tax category you're using in your products.