Hi. I think it depends on shipping. Paying shipping for such a heavy item could be much more costly.
I feed Chicken Soup For the Dog Lovers Soul. It's not "the best" quality out there, but it's quality is between high/mid range and very reasonably priced compared to most other high qualities. It's not widely available locally, on their website they have a dealer locator you can check. Amazon also carries it, usually at a comparable price from what I've found here. They have some better quality food as well. If you buy direct from them they usually have free shipping on items over $25.
Sorry, I can't be of more help. I stick to the same brand so my experience in limiting.
If you search for dog food rankings and find something that meets your criteria you can do a search to compare prices. Here's some websites you can start with:
Dog Food Reviews - Dry Dog Foods - Powered by ReviewPost
Dog Food Comparison Chart - Dog Food Ratings of Premium Dog Food and Supermarket
Dog food ratings, reviews, nutrition, ingredients and information on major brands (Science Diet, Iams, Pedigree, Nutro, Eukanuba, etc.)