Just like our local money, bitcoin doesn't have a definite value for anytime. It will always change from time to time. There's no such thing as FINAL PRICE for bitcoin.Once all the 21 million bitcoins were already mined, there will always be a change in its value depending on its demand, the supply (I mean, the people who sell available their bitcoins), and other factors e.g. big companies holding numbers of bitcoins, countries accepting it, etc.