What's the cheapest way to get from China (Qingdao area) to South Korea?

Personally, it is cheaper to take the direct plane, which offers great discounts from time to time such as flight no. CA4617.

Thanks,

What airline is that with?
Would you not recommend the ferry?

The CA4617 is a flight of Air China.

I seldomly use the ferries, because the equipments and condition is not so good on the ferry, although sometimes, the ferry fare is cheaper.

As I know, the ferries are available from QD to Inchon. The fare for the economical class is around 750RMB per person. The business class costs around 890RMB. The VIP room costs around 1090RMB per person.
The economical class contains 50 beds in one roon. The business one contains 8 people in one and the VIP one contains two beds in one room.