You should probably be able to get a ticket from Seoul for around 330,000-450,000 Won on Korean Air, Asiana, or China Air. (Today 1184.00 Won = $1.00 US)

If you are in Seoul I would suggest looking in the Korea Times or Korea Herald as always have lots of ads for travel agents with prices listed.

Here are a few links to travel agents who have list prices - it should be a good indication of what you should expect to pay. You might also contact the airlines directly at their Korean ticket offices. I think the USO also arranges airline tickets (and should be able to serve you in English) on their calendar page they list Beijing with 3 nights hotel at $620.00US / $425.00 US (I'm assuming the higher cost is for single?)

Travel agents: http://www.boranet.com/
or http://www.timemachine.co.kr/gift_air_e.html
or http://www.skiexpress.com/travelnews/tnews/tnews.htm

Phone Book (to look up airlines) http://www.yellowpages.co.kr

USO Info http://www.uso-center.org/

I haven't used any of these agents so I can't comment on their service.
Good Luck,
-Ty