Proper halloumi is made with sheep and goat milk, which is more expensive to produce than cows.
We have aldi and Costco halloumi in our fridge, both of which costs only slightly more than £1.50 a pack but I don't think either counts as 'proper' halloumi as the Costco version says made with a mix of cow, goat and sheep milk without specifying the percentages and aldi says unspecified mix of milk, so both could be mostly cows milk for all I know.
Predictably Waitrose sells a variety of proper halloumi, but at twice the price. I'm quite happy with our cheap halloumi but if I get to a Waitrose (there's only one in my city and all the other supermarkets are closer) I could buy some of the good stuff to compare.