I dont know if there is anything in this universe that exists with a definite value or price. Even the dollars and other currencies get up and down. Various things gets up and down due to market demand and supply, then how we expect bitcoins price to be fixed ? I guess its very bad if price remains same because then we cannot trade.
Yes, similar to anything in this world bitcoin too have no proper price.
But one thing I want to clear you that bitcoin is such a currency which is going to up and up in price. Hence no wonder different exchange / people follow prices relatively different.
If you take example of other thing like gold it prices are being determined by traders. As world wide it is being traded, different country will have different prices for gold. Similarly bitcoin is being traded my many exchanges and based on bid ask volume bitcoin prices get determined.