Why isn't it "Du sagst das"?
Both are possible. By inversing the word order, you put more emphasis on the das as first element – to underline criticism or disagreement, for instance.
What's wrong with "That's what you say!"?
Nothing; I think it's an excellent translation to convey the meaning of fronting the object in German and moving the subject to the end.
If you come across this exercise again, mark it as "My translation should be accepted".
Would Du sagst das be wrong?
What does this mean? In English this has very little meaning, at least on the surface.