From Erlian city to Ulanbator, you can take the train K3, K23 departing from Beijing, or 4652/4653 leaving from Hohhot. I know K23 departs from Beijing Railway Station on Saturday weekly and arrives at Ulan Bator on Sunday. It costs around CNY1790 for one deluxe-sleeper from Beijing to Ulanbator on K23. For the detailed cost, you would better consult at the train station or contact some international travel agencies.