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It may not be proper, but I'd say it fits for common language. Alternative would be "How are you able to sell for such a low cost?", but that's almost double the number of words.

Could’ve said; why are the prices so low? Not so different.

It just sounds weird to me, because I think I was thought this is wrong a long time ago in school.

Unless it's just because it's early, I can't find anything wrong with it and I've been reading it over for minutes now trying to find what someone might think is wrong with it.

EDIT: As an American, "Why is it so cheap?", and any variant, is completely normal.

> Why is it so cheap

That seems different ; ‘it’ refers to the goods, not ‘price’.

Didn’t know it was common, like said, not native english.

Not sure on the technicalities but as a brit I'd read that without pause, it's perhaps informal at worst.

I am a native English speaker and that is a grammatical sentence.

It is