If I recall correctly, you should expect to feed 20 lb of feed per bird.
So...
25 birds x 20 lb/bird = 500 lb of feed total
500 lb / 50 lb/bag = 10 bags of feed
$18 /bag of feed x 10 bags = $180 of feed cost total
$180 / 25 birds = $7.20 / bird (just for food)