Those were the real value of bitcoin, It's just that bitcoin's price differ from one exchange to the other. Say for example polionex may reach 15K USD first but on other exchanges It is still at 14K+ but that does not mean that polionex is better than the other. If you are already on one exchange such as GDAX then that is the platform where you should look at to check the value of BTC.
I think it all depends on the level of confidence a person to an Exchange, because the higher the level of confidence a person against an exchange then it can make the exchange has a large volume in the sale and buy Bitcoin. so in my opinion you do not think about it, because if you are convinced of an exchange then you can use it continuously. oh yes if you are looking to benefit from arbitrage then you should think that there are some costs you have to calculate, the cost is usually referred to as withdraw fee. You must calculate it as best as possible because if you are wrong then you could lose.