At present, there are three types of Octopus Card for residents and visitors, including On-Loan Octopus, Sold Octopus and Airport Express Travel Pass. On-loan Octopus card is the most ordinary octopus card. You are required to pay a refundable deposit that covers the card cost and possible negative value.

For the detailed information, you can read the above content on this webpage. It is hard to evaluate the value of the cost , but one of my friend said that one may cost at least HKD200 with the Octopus card for 4 days in HK. You can refer to this to deposit your card. Well, the card can be reloaded.