Says you
"Says you" is a specific case of a more general way of refusing to agree. When a listener disagrees with a proposition, he may amend it by attributing it to a source. For instance, if you say, It is good to buy hats
"It is good to buy hats", I might respond, "Says the haberdasher" Says the haberdasher
(someone who makes and sells hats). This indicates that I don't necessarily agree that it is good to buy hats, but I agree the haberdasher says that it is good to buy hats.
Because this is an unusual word order (the verb says
"says" precedes the subject the haberdasher
"the haberdasher") it's not uncommon for people to conjugate the verb improperly. As you noted in your question, you
"you" would ordinarily imply say
"say" rather than says
"says".