I wish I could find a good discount travel website in Korea, like expedia. I'm sure they exist, they're just in Korean. Anyway...so here's what I do when I'm trying to figure out fares. Check out Incheon airport's departing flight schedules to find out which airlines fly to Beijing and when. Here's the link http://www.airport.or.kr/
Then you can just google each airline and find fares to your destination. It's really tedious to do this, but it's the only way I've been able to compare rates so far. And by the way...the Chinese airlines often don't have fare information on the English website. You're probably better off going to a travel agent. But you should check out a couple fares the long tedious way anyhow, just to get an idea.