Originally Posted by Jay71

I rented the egg from KT Corp which was 5000W/day (if you reserve ahead you don't have to pay for the device rental, just the daily data fee). It's unlimited data so you're looking at 15000W to 25000W.

Conversely, you can get a prepaid sim. Per the PrePaid Data Sim Wiki, you're looking at the following 30day options from KT:
500 MB: 8,800 won
1 GB: 14,300 won
2 GB: 19,800 won