It is extremely important to make sure you and the taxi driver (or private car driver) to agree on a negotiated rate (if you are traveling long distance) before you get in the car. I have heard stories of drivers ripping people off refuse to let the passengers out unless they give them more money.
On my previous trip, my friend Kathy and myself wanted to visit the Seongsan Ilchulbong (Sunrise Peak) for the sunrise on our last morning in Jeju and our friendly hostel host was able to negotiate a rate for us. Our negotiated rate was ₩60,000 (about $57USD) for the 2 hour round trip. After returning to our hostel, I noticed that had we not had a negotiated rate, we would have paid close to ₩85,000 (about $81USD).
So do yourself a favor if you are staying in a hostel, check your your host if they can negotiate a good rate for you.