Coffee in a coffee shop is expensive throughout Seoul. It is the same price in Gangam as it is in other places in Seoul, so its not like there is a different rate for Gangnam. A Starbucks americano is likely to be 5000 won, which is roughly 4.5 USD. There are different levels of coffee shops, and hence the price is likely to range between 3-8$ depending upon where you go.
I think the basis for your question is the Gangnam style song, where the girl is described as someone who loves to have Coffee. It is a common sight in Korea to see well dressed girls sitting around in coffee shops with their laptops and a cup of coffee. It is not so much for the coffee as much as it is for the feeling of being modern, independent and career oriented. So when PSY says "Coffee drinking girl", it doesn't necessarily refer to the affordability of the coffee but to that attitude of wanting to spend time in a coffee shop. Also,there is a term in Korean which refers to girls who skip their lunch, in order to spend that money on a cup of coffee.
Source: I have been living in Seoul for the past 1.5 years.