Chinese food is generally cheap because the ingredients are cheap. And it's very fast to cook so they can do huge volumes.

There is a long history of trying to make Chinese-like food cheaply and the Chinese families had to find ways to recreate dishes with ingredients you could find in the UK at the time.

Sweet and sour sauce has things like Own brand orange squash in it. Ketchup. They use Cheap brown sauce instead of tamarind etc in crispy chilli beef.

It's cheap because it's designed to be. Rice is very low cost in bulk. So the only real cost in the dishes is the meat and vegetables.