We feed organic feed, about $32 per 40lbs bag. I just ran the math and during this past winter (no forage available), it cost us $3.96 to feed each chicken a month. We have a few silkies, one tinier bantam and the rest are all large fowl (regular sized chickens). Brahmas are particularly large chickens though, so I imagine they would need to eat a bit more. Temperatures this past winter were warmer than normal, but averaged around 20F. We do not heat the coop in any way, but do use a de-icer in their water nipple bucket (heating a coop could in theory reduce feed usage, if the coop was warm enough, not worth the fire risk or cost, IMO).
The previous winter I think our feed cost was $2.54 per chicken per month. I am hazy in memory about how many chickens we had that winter. In any case, regardless of the number of chickens we had that winter, our total feed cost was about 18% less (and it was colder that winter over all). Not sure why that was, unless we had fewer chickens than I recall.