Simple answer, buy the more expensive dog food. I spend roughly $50/50lb per bag of 0-grain feed that I give my dog. This lasts 2-3 months for a 40-50 lb dog.
Completely OT, but holy $#!+ that's cheap! I pay $2.50 a lb (GBP45 for a 12kg bag) for dog food that is only 20% meat / meat derivatives, and includes grain. It's far from the most expensive but it's the best I have found at that price.