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.
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.