You are right. But since the OP mentioned that she/he will be staying near Yeongdeungpo-gu subway station and planning to commute to the downtown area, a ride should cost 900 krw (to City Hall station).

To OP, I would not put too much cash on the card at a time. There were plenty of stores and kiosks where you can refill your card pretty much everywhere. After a day or two, I think you will figure out your spending pattern. Others may think different, but I found it pleasant to just walk around on foot at least within downtown (maybe not in summer).