As mentioned, there are not many actual centers that offer English language instruction. Busan scuba is the only one that I know of.
However there is a group of foreign divers here known as Expat Divers in Korea (search them on facebook). The leader of the group is Tim and a PADI MSDT and based in Seoul. He typically teaches a few certs or specialties every year to maintain his instructor status. Mainly we organize dive trips around Korea. Not that when I say "organize", I mean we are a club and just pick a dive location and create a facebook event and dive. There are no organizer fees and the only costs are the actual dive fees.
With that in mind let me break down some of the pricing structure here.
Typical Diving costs (if you have all your own gear) are about 35,000-45,000 won per dive. This if you are basically just like a typical person with your gear, certified and just fun diving.
Gear Rentals - Usually between 30,000-40,000 to rent a full set of gear. Individual pieces are typically 8,000-10,000 each.
Diving with a guide - If you are diving with a guide or some shop that basically arranges with the actual dive operation, they typically charge 60,000-70,000 per dive. If I recall correctly I paid 140,000 a few years ago to Busan Scuba for a tour they had arranged in Yeongduk. I speak Korean, drove myself there and had all of my own gear...but because someone else had arranged it there is their "guide" fee. Not a bad deal however if you prefer it all done for you.
Clubs like Expat Divers do it just because we want to have fun diving with foreigners. We are not a business so we don't need to charge fees. Tim and I are just good friends and decide to take a trip, and invite anyone else to come along if they like. That's it. Actually a lot of times it is cheaper with us because Tim as an instructor often gets diving fees waived. However rather than him paying nothing, we just average the costs for all divers.
For myself, I am based in Daegu and an SDI Divemaster. I do most of my diving in Pohang where I manage ADS 126 for the Project AWARE Dive Against Debris. Most of my diving is underwater trash removal and I do that typically 1-2 times per month. I speak Korean and act pretty much as the logistics coordinator for Expat Divers when we arrange a dive trip.
Cheers to all divers out here in S. Korea. Feel free to contact myself or Tim if you are interested in doing some fun diving ( or Dive Against Debris cleanup events).
https://www.facebook.com/groups/552679698230761/