Well, most expensive macro will always be protein, with fat coming in second (good quality fat) and carbs 3rd.
This may sound like the most obvious answer but i’d go with a reasonable amount of whey concentrate/isolate and mix it in cream of wheat/rice with some berrys/nut butter.
You won’t be able to beat the economics of whey protein when you calculate the grams of protein/$ if you go for reasonable brands.
Besides that- obviously – costco chickens and fish