I saw an advertising poster on a Samsung shop that was saying "iPhone 5, because you have more money than sense".
But I don't get it, is that even a correct sentence?
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It is grammatically correct, but it is a phrase, not a sentence. It is saying that you have more money than you have sense (which as Zairja said in the above comments, "sound mental capacity and understanding typically marked by shrewdness and practicality", couldn't have put it better myself).
An important thing to note about this is that there is a difference between something being grammatically correct, and being a complete sentence. It is merely a subordinate clause, but a grammatically correct one. In the case of advertisement, this is commonly done. The part that's "missing" for it to be a complete sentence is an implied "buy the iPhone 5", or even "you would buy the iPhone 5".
No, it's a sentence fragment. It's short for something like "You should buy an iPhone 5, because you have more money than sense."