South Korea has 2 GSM providers, KT Freetel and SK Telecom. I don't know about their rates however. The prepaid card are rechargeable, however they have a limited time validity, the one I had in HK expired 6 months after. If you recharge the phone you have a new validity that is between 60 days and 1 year, it depends of the provider and the value of the card you are buying.

GSM phones are quite inexpensive in Asia, you can find something for less than an hundred that will be unlock.