I did this calculation a while ago (didn't include hot dogs, because 🤮). But whey protein absolutely stomped every other protein source in terms of cost effectiveness. This really isn't surprising considering it is a dehydrated, shelf stable source of pretty much pure protein, which also (iirc) is a waste product of cheese making. So you are basically buying something they want to give away for free, which has no cost to keep it cool, no need to move product before it spoils, no additional weight of water or bone to transport. They just add some flavoring and sweetener and bam! You're jacked!

You can also just not eat meat very often to help keep costs down. For the 2 of us this week we have a single pack of 600g which is above average for us.

Sometimes get tinned mackerel which is much less total meat, but it's got a stronger flavour than chicken or pork so it can go further in a meal. I would look at catching crabs from the harbour but my partner refuses to eat them.

as convenience foods go, 2/$1 gas station hot dogs exceed 500kcal. nothing comes close.