It roughly means, "is that so?" or "really!" in response to some surprising news. I'm not sure if I can explain it logically, perhaps "you don't say!" suggests some disbelief? A: "My brother just won a million dollars." B: "You don't say!"

It is often used in a sarcastic way as well, to point out that what the other person said was obvious.

"you don't say" is very very similar to 废话！ More often used as a sarcastic response rather than a genuine expression of surprise: you don't say = no kidding! = no joke! = yes i know!!!

It's an expression of surprise or disbelief. A bit like 哪裡哪裡。

I think it still depends on the context 不见得吧！是吗！是真的嘛！