You don't have to worry much about paying for waiting time if you lock a deal in in advance (or during the ride up to Mutianyu). I'd like to think that you could pull off the entire circuit for around Y450, maybe even Y400, but I'm not so sure anymore (the economics of driving and wages have changed quite a bit since we first started these GW driver threads ~10 years ago).

If you really want to take the 906 bus back, it is a viable option, but I think you're overlooking a few logistical issues:
1) Dongzhimen is basically in the center of the city (not quite, but close enough) whereas your hotel is on N. 4th Ring
2) The bus stations are not especially close to the subway station, and it's not a very pleasant walk in any event... so taking a taxi at that point --or from further out if there is a stop-- would be worthy of consideration (this shouldn't be more than about Y35)
3) You could also scrap the Mutianyu plan, go straight to your hotel from the airport, and do Badaling (train from Xizhimen) instead... while most of us prefer other wall spots to Badaling, it is the most accessible via public transport