I have a question. Why do two-bedroom apartments near Gangnam Station seem cheaper than expected?

I am looking to rent a two-bedroom (or three-bedroom) apartment on a "monthly rent" basis in Seoul.
Why does Gangnam not seem more expensive than areas like Hongdae, Hapjeong, or Sinchon?
The area between Sinnonhyeon Station, Gangnam Station, and Yangjae Station isn't as expensive as I thought. (In fact, it seems even cheaper than the areas further up...)
I am aware that the area near Gangnam Station can sometimes flood during the monsoon season,
but as long as it's not a semi-basement unit, that doesn't seem to be an issue.
Is Hongdae actually expensive??