What does the "Cost per Transaction" chart on BlockChain.info show?
On the chart overview page you find the description "Cost per Transaction: A chart showing miners revenue divided by the number of transactions.".

Meaning, it's the total value of the block reward of a block divided by the number of transactions confirmed on the network in that block.



Here is an example with made up numbers:


Block reward is 25 BTC + 0.1 BTC in transaction fees. 
There are 502 transactions in the block.
Current exchange rate is 367.3 USD/BTC.


25.1 BTC / 502 transactions = 0.05 BTC per transaction

367.30 USD/BTC * 0.05 BTC per transaction = $18.37 per transaction



Concerning what will happen after the regular block reward is reduced/discontinued, check out:


Will Bitcoin suffer from a mining Tragedy of the Commons when mining fees drop to zero?
How much will transaction fees eventually be?
What happens once the mining reward gets cut in half?