If the strike price of an call option is 30$, so finally the stock price is
40$Is the payoff of the option 10$?
I doubt it.

The guy who bought the stock or the call option would both bear the same
risk of the stock, but if the guy bought the option at first,  he would get
 some extra money (the interests of the stock price now minus the price of
the option).

So the payoff of a call option maybe is max[St-K+r, r] instead of max[St-K,
0]

where St is the final price of the stock, K is the strike price, r is
the the interests of the stock price now minus the price of the option

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