My kefir grains seem to have adapted well to powdered skim milk. Currently I'm making up an evening treat for 25 chickens of about half a litre of kefir, half a bag of rolled steamed oats and anything else I wish to add by way of protein (sometimes cooked lupins, sometimes a little fresh mince or high protein food scraps). This gives about a quarter of a cup of afternoon treat per bird — a quarter of each bird's daily ration, except the meat hybrids, which wolf down more overall.
Skim powdered is quite cheap at around $6.50 for powder to make 10 litres. Ten litres of kefir goes a long way when used as a supplement (around 20 days). The oats as I've said earlier are $1.19 per 900g bag, and I use half a bag per day. In total for this supplement for 25 birds I'm paying just under $1 per day, or $1.50 if I add about 250g of pet mince (which I do roughly every 2nd day, not all the time).
I've also found a new butcher selling pet mince without preservatives for $2 per kilo. Again, using this as a supplement rather than a whole-protein source, it's not a bad price and it also isn't terribly fatty (a thing to avoid with mince). The butcher assures me it's lungs, hearts, offal etc with fat added for texture, rather than just fatty scraps.
This is most definitely not the 'cheap' way to feed chickens. In the old days I would have had a dairy and plenty of meaty bones and other farm scraps. The chickens would have been out all day picking up horse dung and finding plenty of B vitamins and probiotics into the bargain. And mostly the chickens would have been foraging for themselves, so they would have thrived on little more than grain.
When we move out of this block with its huge overshadowing tall trees and zero understorey (perfect prey territory for goshawks), I'm going to free range more, and that will naturally cut costs. However for now the plan remains the same: keep feeding an all-natural diet, aiming to supply essential amino acids and minerals in as natural a way as possible; and watch the birds like (but only a little bit like) a hawk. :)