A day pass for the MTR costs HK$ 55 and a same-day round trip ticket on the airport express costs HK$ 100 so a total of about US$ 20.
I took a Big Bus tour in HK a few days ago. There are three routes: two on HK Island and one around Kowloon. An individual trip costs HK$ 200, or you can get a ticket for all three, including the cost of the Star Ferry and the Peak Tram for HK$ 350. I bought a ticket for the Red Line only. It was okay, but if you do a little research ahead of time, it would probably be better just to use the subway, which is fairly easy to use.