What does the "Cost per Transaction" chart on BlockChain.info show? According to the Blockchain.info: Cost per Transaction, the latest "Cost per Transaction" value is 16 USD. Apparently, it's not the transaction fee, what does it mean then?
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?