The lowest-priced, decent food I know of is Kirkland (Costco) chicken and rice -- $28 per 40 pounds. It's nearly identical to 4Health (Tractor Supply) and Diamond Naturals (same manufacturer), which cost just over $30 per 40 lb. It's very economical because of the amount you feed (around 3 c. for an adult), and most dogs with no allergies do quite well on it. It's the lowest-priced option I know of that I'm comfortable recommending -- we've fed a lot of in in our breed rescue, and it feeds out nicely. There are better foods, but not for this price.
Victor's entry-level options and Fromm Classic would be the next step up in cost.