There is a surprising lack of information in English on actually getting prepaid SIM cards for long-term use in Seoul, not just as a tourist.
There is a shop near Yeongdeungpo Station crossroads, next to the Olive Young shop, that handles “Mobing” prepaid SIM cards, and they have an open pricing chart.
For me, the last KT prepaid package option with 10GB data seemed to be the best option. (“모빙 선불데이터10G” at 33000 won approx. US$30)