English word order changes when you are asking a question, "is" must come before "this".
아주머니 is polite, but if you call a person you have never met 아줌마, she's not gonna like it.
Hush now, young grasshopper. Don't question it. Be happy with the price and go.
Because they havent used the word for inexpensive '싼'. instead they used 안 not + 비싼 expessive so they're not reallly the same.
I think my answer: "Lady, why is this camera not expensive?" should have been accepted.
We don't use lady in this way in regular spoken English. It would be a little condescending.