For unlimited GPRS it cost HKD 138/month. Since everytime I top up HKD300 will get HKD400 top up value. The actual pay was HKD103.50/month.
(per usage pay was HKD0.04/kB. If I didn't used unlimited, it will cost me HKD24000/month for my monthly usage)
No any other cost involved. But you know, GPRS was slow compare with broadband.
I used seperated SIM & mobile for my call phone and data service. Because if used one SIM, while you are download/send data, the others people can't reach you.