Hi there!
I just want to know that, does the usage of Wifi costs me anything?
like will my carrier bill me for the usage of Wifi, like it does for using gprs

if you are using your wifi at home or at the office it does not cost anything ;)

if you are using your carrier's wifi network (e.g. hk csl has covered hong kong completely with wifi hotspots) so you will be charged by your carrier. however, most carriers require to subscribe to a special wifi plan in order to be able to use their wifi hotspots.

I agree with funkym. If you are using your own Wi-Fi hotspot/network, the only charges you would incur would be those coming from your ISP. With Wi-Fi, you should also be able to save a significant amount of data (if you are not on an unlimited BB data plan) as all BB data will go through it.