So why the generalisation of quality food always being very cheap?

Because I was not talking about meat only and there is a ton of cheap stuff even in supermarkets that is very healthy... like raw peanuts for example.

This is incorrect, many people build natural muscle eating awfully but keeping a high protein intake. Have you never heard of a dirty bulk?

You have to be very careful what you're talking about because a huge number of people use steroids and hide it. I've met a lot of those. Also, the line that divides what is "dirty" or "clean" is very thin because you can also make food that is commonly considered "dirty" in a healthy way... I make my own burgers and this is extremely different from McDonalds and such.

to buy the “healthier” versions of a lot of main ingredients (especially meat) is always more expensive than the less healthier alternatives.

Here's the thing... What is really expensive is readymade food. If we are to put this on a scale from cheap to expensive it's like:

"non-organic"---"organic"-----------------------------------------junk...

if you're making your own food and not buying crap you're already saving a lot, so it doesn't even apply. Like I replied to op in another comment, I just feel too many people neglect how cheap and necessary cooking is... and for pure ignorance.