it cost a friend of mine 70k to Gangnam recently via taxi. (this is before the slightly higher charge after 12am kicks in)

But there are some late night buses after 10pm....as I took one at 1.20am to Express Bus Terminal also fairly recently.

Can't recall the exact times but there are definitely at least 3 late buses (limited times) from bus stop 5A to:

Seoul station
Gangnam Express Bus Terminal
Yeongdeungpo station

And as someone mentioned previously, KE has special buses for late arriving flights.