37-it didn't cost you nothing

[JaneP]

What Sebaylias says is correct. It can't be classed as improper English - because it is spoken and understood by a fairly large community and therefore has worked its way into English. However it should be avoided in any formal context. It would also sound very strange to hear a non-native English speaker saying it!

[Sebaylias]

It would also sound very strange to hear a non-native English speaker saying it!

It sure would!!!!! It would sound absolutely ridiculous!!!! I'm laughing here just imagining a SLS saying it with an accent

[ewelina]

To me the sentence "It doesn't cost nothing" has nothing to do with the logic of a sentence, but more with the emphasis of its meaning.
However, I may be mistaken: as a Pole I am used to double negatives.

In fact, I rather like what musicians, subcultures and minorities do with the language. They try something new, they still search. To me, it is just another way to express artistic or individual freedom and integrity. It also means that language is a living and thriving thing.

It also means a lot of trouble for non native speakers f.i. "it don't mean anything"!