I have found that adopters are generally open to information about food when I show them spreadsheets and information on the foods and why they are ones I use. I am pretty detailed about it, and while I can't say that years later, they are sticking with them, initially (and I send food with the dog) at least they are!
I do a calorie comparison, a nutrient density type comparison, and cost. So that they can see that spending $____ on Bag A is actually less expensive than Bag B.
I give them a bigger list than just 2 higher priced foods though. I usually list all the foods the dog has tried while with me, and list brands/types that seem like they would work for the dog. I also make sure the food is available to them within a reasonable distance. Of course, my adopters have all been kind of OCD too so it makes it easier.