How to go to Metropark Kowloon Hotel in Hong Kong?

I'll be travelling to HK with my family of 5adults and 2children. This will be our first time to HK. I have booked Metropark Hotel Kowloon so I want to ask how to go there the cheapest way. I read some recommendation by Airport Express and then free shuttle K5 direct to the hotel. But it cost hkd 90/person. Is there any cheepest transportation for us? Can bua A21 take us to the hotel? If possible how much is the fare and where is the stop nearest the hotel? Thank you