The taxi fare will be about CNY50. Why don't you take the bus No.109, which could drive you from the west train station to Dongsi station directly? It is only CNY1, and could be found on the train station's south square.

On the other hand, at the north square of the train station, you could take the bus 特2 to Chongwenmen, where you could transfer to the subway Line 5 to Dongsi station as well. The fare is only CNY3.5/adult.