Calculating network fee

Can anyone please tell me how to reliably calculate the network fee at any given time?

I've used etherscan gas tracker and used the current gwei to calculate the price, but it is out by a factor of 100 from what I am quoted by CoinBase.

21000 * 8 = 168000 = 0.000168 ETH = £0.49

CB total cost = £40 / £20 of this is network fee

I simply just do not understand how to calculate this fee or the total cost and CoinBase aren't much help.