The general formula to convert from housing deposit to monthly rent is $10,000 deposit equals $50 monthly rent. (Until recently it was $100 rent). It's possible to find lower, but $10,000 is usually the minimum deposit.
Housing costs will vary greatly depending on location. $90,000 is in Gimpo, just outside of Seoul. Busan is probably half the cost of Seoul, and outside any major metro areas it's probably even lower.
Also even within the same real estate market prices vary greatly depending on the neighborhood, type, and size of unit. Another major factor is distance to nearest public transportation (subway).
Another thing to note is since monthly rent is usually more profitable for landowners, there is a movement away from the cheonsae system. However, I was still able to find plenty of options exactly one year ago.
I wouldn't do it unless I had a partner who is already a citizen , but I'll definitely keep this in mind.
How much is rent if I didn't want to deposit that much ?