I have read so many accounts on bus travel to Mutianyu.

Marion, you need to take no.916 Express from Dongzhimen Transportation Hub. It departs from 05:50 to 19:50. The bus can't take you to the wall directly. You need to get off at Huairou North Street. Then you can share a minivan with other travelers to reach the wall.

Did you mean taking a minbus from Huairou North Street to the Wall? If so, the cost for the mini bus is around CNY40 to 50. It takes around 20 minutes to reach the wall. If you plan to hire a minibus from Beijing to the wall, he cost will be much higher. A round trip may cost CNY400 at least.