The Seoul Metropolitan area's public transportation is very convenient. With one public transit card (T-Money Card), you can use all forms of transportation (subway, bus, taxi). Not only that, public transportation is cheaper using a transit card. The base fare for subway is 1,250 KRW (approx 1 USD), and the bus is 1,200 KRW (approx 1 USD), which varies by city and the type of transportation. The rates increase based on the distance, and it has a transfer system between Subway and Buses so that you do not have to pay twice for base fare. Suppose you use the subway to go to Seoul Station, it will take about 90 mins, while it takes an hour if you took a bus.
To learn more about the Seoul subway network, check out