I have to say one of the best cheapest foods I've fed is Diamond. I think it was around $25/40lbs. Not a bad price.

I'm currently feeding Canidae. It's an all natural human grade food and they love it! I think it's around $40/40lbs.

I've also fed Eagle Pack, another all natural human grade food. I think it was a little more expensive, $40/33lbs.

Good luck on finding a food!