You can park your car at city gate car park in Tung Chung, thats about as close as you are going to get to DB thats easilly accessed by bus.
Not sure what the monthly parking rates there are, but its probably going to be somewhere between $1200 and $2000 per month, with a min of 3 months paid up front.
Some people here will argue against having a car, but to be honest, we have seen so much more of Hong Kong by having a car on the rare occasions we use it recreationally.
Visiting places like Brides Pools, Plover Cove, Tai Tong Lychee Valley, which as a family makes day trips a lot simpler, allowing you to carry things like deck chairs, esky or if you have young kidlets, strollers and everything you need for a pleasant day out.