Hi!
I hope this is useful to you. However, I don't know how well the staff in communicating in English (as this bus mainly used by mainland China or Hong Kong people), and personally I have quite bad experience in communicating in English outside hotel in mainland anyway.

http://ctsbus.hkcts.com/routes/hk/hk_dongguan.html

You need to change bus after crossing the boarder and depends on schedule there will be some 30mins or so waiting on the bus. But if you gonna stay in Sheraton, it is the first stop HK->DG and the last stop from DG->HK, so very convenient.

$100/OW or $150/RT HKD.