The minimum amount of money you have to hand over is $150 ($50 deposit + $100), which you will need all of anyway if you use the Airport Express to HK. So you should probably top it up to $200 (+$50 deposit). In any case you get back all the unused value and the deposit when you return the card at the airport on your way out.
http://www.octopus.com.hk/eng/index.jsp