I don't know about letters, but I do know that it costs 4.5 yuan to send a postcard from Beijing to the US
See related link. Answer their questions and out pops a number. I got 94 cents for a normal letter (under 1 ounce) to China.
1 to 14 days or more
According to Royal Mail it starts at 56p - see attached link for additional info
China is a great business partner.