I recommend Olleh's prepaid service. I visited my family in Korea this summer, and used Olleh's plan.
I carried my iphone 5, and bought a sim card ($9) and bought 1gb data ($10) & calling/receiving plan ($10). If you make a phone call, it's very expensive - let others call you after texting them because receiving phone calls do not get charged.
Total was thus around $30 inc. tax. Other companies only provide expensive plans. I used it well during my stay (3 weeks).
Hope this helps somehow.