In my recent solo trip to Korea, a 14-day itinerary, my hunt for a budget data SIM card, and optionally with voice calls, revealed pricing that way exceeded my ideal budget. For a Korean SIM card: $56 for 15-days for unlimited data and voice calls, or otherwise a $77 for a 30-day unlimited SIM card?!
My budget was only $15-25 and I perhaps required roughly 2 GB of data for the 14 days, mainly for navigating around on map application, and googling on-the-go. So here, I would like to recommend something else (not only limited to travelling to South Korea)