It's more convenient if you'll use the octupus card rather than riding a taxi or pay and wait for line each time you'll go to certain place. For the adult the octupus card is $100HK $50 deposit which is refundable in case you don't consume all. Also, you can use the card in other stores like 7-11 convenience store, McDonalds and others. For kids I don't remember the exact price but it's much cheaper.