If you want a cheap flight from Bangkok to Beijing, you've got zero airlines to choose from.
Beijing Capital Int. Airport is just one of the airports in Beijing. If you're not set on a specific airport, try changing your search from Beijing Capital Int. Airport to Beijing to see a broader range of flights – they might be cheaper. Airport.
If you're planning on going to Beijing during this time, you could take a look at other airport options. Or you could try to get a last-minute bargain.