Someone posted about how renderers extract the fat from municipal
sewage ponds (all the oils that leech out of the turds and float to
the top) and add that to cat food.  I'll bet you wont' find any sewage
fat in these expensive brands.

But, sewage fat is wholesome nutrition once the pathogens have been
removed.  As Bismarck said, people are better off not knowing the
story.

I really believe the catfood story is less clear than many seem to
believe... my impression is that cats can go a long time on dry food,
wet food, cheap food, expensive food.  The main thing is to monitor
the health of the cat and be alert to any problems caused by diet, and
modify for special needs.  For example, my cat had urinary crystals
when I took him in a few months ago.  I am giving him Friskies
"special diet" meant to balance urinary pH and prevent that, and soon
I will take him back to the vet to make sure it's working.

Charlie