The real problem is that when miners are competitive they'll always add cost (in electricity, equipment and whatever) up to the level of the revenues. So if you have a block reward - it will approximate the cost of the system.
There is a famous post on this subject: Nothing is Cheaper than Proof of Work http://www.truthcoin.info/blog/pow-cheapest/ (even though I believe that Proof of Stake can be cheaper: https://medium.com/@zby/proof-of-stake-can-be-cheaper-than-p... - by reducing the block reward).
