What determines bitcoin pricing? (ex: the current market for bitcoins is 16billions USD$. If someone were to buy coins until he spent 8billions USD$, does it means coins will be worth 50% more?)
Also is it better to buy, mine or rent a miner?
The price depends on supply and demand.
If you can buy mining equipment for less than the number of bitcoins it will mine, then you should mine. Otherwise, you should buy.