Ethereum plans to shift from PoW to PoS and there'll be no mining involved anymore. The incentive for peers to participate in the network and confirm the transactions will be the transaction fee. How much will it be?
Good question, and one which is discussed in the latest post on the "official" blog:
One of the main problems discussed is:
Let us suppose that relying purely on current transaction fees is insufficient to secure the network. There are two ways to raise more revenue. One is to increase transaction fees by constraining supply to below efficient levels, and the other is to add inflation. How do we choose which one, or what proportions of both, to use?
The article then discusses how to address this, without drawing any firm conclusions. So in answer to your question: I think this is yet to be decided.