Also from who you get the bought hash power?
Buying GH/s on cex.io will get you to mine instantly. I guess it depends a bit on luck, as you're given dividends based on the amount of GH/s you have versus the blocks that cex.io solves.
You're basically buying your GH/s from other members who are selling theirs, though everything is ultimately "owned" by cex.io since they have the hardware.
hm, i'm still having a hard time understanding the point of this and how does one make profit
Profit comes with trading and a little luck. No big mining equation, nothing... Just trading.