As ColinFine says, it is not standard or correct grammar. It's slang.
It means, "So you say" or "That is what you say". The implication is that the statement in question is not true, and the only proof that the person is offering is his own assertion that it is true.
For example:
You Ruritanians are all stupid!
Says you!
The phrase is very informal. It is commonly used in verbal arguments. You would not use it in a scholarly paper -- except to add a humorous touch.