if you are looking to get the really cheap flights from Bangkok to Beijing you need to be a little flexible. The cheapest flight and discount airfares are on Air Asia. Here is the problem Air Asia does not run flights to Beijing, so what you need to do is fly into Xiamen and then get a connecting internal flight to Beijing China. There are 6 different chinese airlines to chose from.

I am not sure on the China Visas if flying into Macau will work. A note, you must have a China Visa in your Passport before you get to China. I found that the Chinese visa was a little expensive at $52 from Phnom Penh, but they can give you a 30, 60 or 90 day visa for this price, so if you think that you might want to stay longer remember this when you apply for visa .This strategy will work for Shanghai and Chengdu as well.
Cheap Charlies Hotels don't aim to please the airlines, We aim to please you by providing you with really Cheap International Flights & discount Airfares. We will be focusing on the Asia Oceanic major tourist cities like Bangkok Phnom Penh Saigon Sydney Manila Waikiki Beach.
I have found the really cheapest and discounted flights using the airfare search engines. The best seem to be Sidestep, Kayak, Mobissimo and FareChase for the cheaper international airfares .The new ones that you can use to check the flight air fare rates against are FareCast and FareCompare which use forecasting logarithms to give you the best idea of what the airfares should be in the range of from x to y price.