what percentage of Chinese exports are bound for the US, valuewise? I’m trying to determine just how important the US is to the Chinese economy. On researching this question, my biggest discovery is that I don’t understand economic figures and/or have not found the right source.
Other discoveries:
China mainly exports low-value, labor-intensive items while importing high-tech, high-value stuff.
Some of the exported volume goes through interim ports, confusing the issue.
Does anyone know a source that would help fight my economics ignorance?
The U.S. accounted for about 21% of Chinese exports last year, according to the invaluable CIA World Factbook.
Wow. that’s way below what I thought it would be. 21% is still significant, though. And it is the largest trade partner, but still I thought that the US was something like more than 50% of China’s export value. Even if Hong Kongs 16% is added, it’s still short of that level.
Keep in mind that with few exceptions, the US is EVERYONE’s biggest trading partner. 
you could also look at the (re) exports from HK to the US. Any number of economists calculate the “true value” of what is “really” shipped from China to the US in different ways and get wildly different answers.
Even official trade numbers are pretty inexact. If you try to balance all the different country’s exports and imports, it’s never close.
That seems to be one of the main problems I’m having in this search. However, the difference between my expectations and the Factbook numbers is great enough that I still feel that my assumptions were waaaaay off.
Tangentially, why is Hong Kong still counted separately? Aren’t they part of the PRC now? Or is there some kind of special trade situation?
21% is still a huge chunk of the Chinese economy. Remember, as you said, they are producing low-profit, commoditized, labor-intensive products and services (“goods”). The more commoditized goods becomes, the harder it becomes to differentiate from another product on the market. 1/5 is also a large amount of people and dollars in the Chinese economy. That more than enough to destabilize any economy and place into a economic downward spiral.