Attached is a subway map(English) of Seoul city. Depending on distance, fare is about $1US to $2US.
You may buy a ticket at subway stations or you can buy a rechargeable(prepaid) subway card(지하철카드) at most convenience stores in Seoul. The card can also be used for buses in Seoul as well as other cities in Korea.
For a larger map, please down load the map and zoom-in.