1) Why was number of coins limited with 21 millions?
Because Satoshi thought so ;P
The only things that decide the number of coins are the 50
BTC initial reward and the halving every 210 000 blocks.
If he would have chosen 56 million you would be asking the same thing:P
2) why is getting coins so expensive, so its above all normalities? (collective usage of power to mine them)
Demand and offer, the basic principle of a free market.
3) what has bitcoin to offer additional to, say PayPal, that makes it so expensive? [/b]
Paypal can block my funds, Paypal can close my account, Paypal might prevent me from creating an account.
Nobody can touch my Bitcoins, nobody can restrict me from spending them how I want where I want.