How much should we initially purchase for an Octopus Card ?

My mom and I are planning to visit HK this October for 4 nights and our hotel is located near Jordan Station. We are planning to visit Disneyland, Ocean Park, Peak Tram, Sky Terrace, Madamme Tussauds, Ngong Ping 360, Aqualuna and I would like to budget our fare for the MTR. Your help will be much appreciated. Thanks!