how much time requires and the cost of the operation to obtain visa in Beijing ?

Dear Ambassdor of Mongolia,
Atten. Visa Office
I am going to visit Mongolia next September via Trans Mongolian train, departure in Beijing, and I will need to obtain visa previously in Beijing (because in my country, Argentina, it isn't an embassy or consulate).
Please, I need to know what things I will need to have, the adress of the visa office in Beijing, how much time requires and the cost of the operation.
Thank you very much. Looking for your answer.
Daniel Crosta