Completely unbeatable value especially factoring in time and convenience and heartiness is picking up a rotisserie chicken and a bag of frozen veggies.
Liver is also very affordable and extremely nutritious.
Meat is not all that expensive for protein and comparatively quick and easy to cook, agreed, but the cheapest option is still veg.
Liver, and offal in general, is much underrated for both taste and health. A lot of people in the UK just do not eat it and its becoming hard to find (liver the common, but other things are not).
Maybe your point is that you spend about as much on meat as you do on produce, but that depends on your specific diet since they are not equivalent food groups.
Calories aren't a good measure of anything beyond burning your food. Unless your body works similar to a steam engine, it really doesn't matter how much energy you can put into water by setting your food on fire.
My point was simply that I spend roughly the cost of quality meat in my area is on par with or cheaper than quality produce given what I need to put into a meal to feel full and satiated. My point isn't that I eat only meat, I don't know the last time I had a meal that didn't also include vegetables or bread, for example. I was only calling out that one doesn't need to stick to vegetables for a reasonably priced healthy meal, at least where I live.