$220 HKD for a one-way Airport Express ride and 3 days unlimited MTR
$300 HKD for a two-way Airport Express ride and 3 days unlimited MTR
Of course the best value is if you are a single passenger and complete a roundtrip between the Airport and Hong Kong station. If you are travelling in a group, MTR has some group discounts for Airport Express rides, see my other post.
Additionally, if you are planning on making many trips between the HK side and Kowloon on the MTR this is a good deal, since usually it’s about $10 per cross harbour trip. You can also take advantage of the unlimited Octopus card for travel to the New Territories and save the shorter trips for after the 3 days.
Everytime I came to HK I would buy this card, it saved me the hassle of adding money onto the Octopus card and really saved me money and I was hopping back and forth between the HK side and TST / Mong Kok to visit all the touristy street markets and sights.
Don’t forget, if you buy online, 10% discount for HSBC cardholders and Asia Miles earnings.
Details here: http://www.mtr.com.hk/eng/fares_tickets/intro.html and here.