Subway in Hongkong is quite cheap in comparison to other world megapolises. From the other side, the process of buying the ticket might turn out to be a bit complicated for a tourist. Buying a single ticket, you need to indicate the station and the direction: the longer the way is, the more expensive it costs. Not to always bother with that, you can buy Octopus card, put money on it and use the subway till you have positive balance on it. When you return the card – you get the left balance back.