In the view "product / listing_price", try to change
echo JText::_('PER_UNIT');
By:
$productClass = hikashop_get('class.product');
$categories = $productClass->getCategories($this->row->product_id);
if(in_array('2',$categories)){
echo JText::_('PER_DAY');
}
else{
echo JText::_('PER_UNIT');
}
and replace '2' by the ID of the category where you need to display "per day" instead of "each".
You have too to add a translation override in HikaShop > Config > Languages, in the override part. PER_DAY="per day"