I would say it's primary meaning is similar to "really?" Person A: I just found $100 on the ground. Person B: You don't say! But it can be used ironically, in the sense of "I'm not surprised." Person A: I locked myself out of my house for the 5th time this month. Person B: You don't say.

I agree that it depends on the tone of voice used. Usually it's a term of surprise, but it can be a sign of total disinterest in reply to what a person is saying.

"You don't say." = 废话！ ("You don't say" 常常有讽刺的味道) Tim 的例子： Person A: I just found $100 on the ground. Person B: You don't say! 在这裡的 You don't say! = 是吗？！ 真的吗？！