I don't think animal products are all that expensive tbh. Chicken, milk, yogurt, eggs are all really cheap if you consider the protien content.
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.
Of the four animal products you list there, only one is meat, and its a cheap meat...
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).
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.