$3.50 is usually the price thrown around for a new mine to be viable. Though I have not seen the economic modelling in detail. Couple of sources repeating that figure:
http://www.kitco.com/news/2018-06-0...de-Wars-Because-Of-Growing-Supply-Issues.html
(at about 8 minutes)
Both of these sources are bullish on copper in the long term, I'm just
personally taking a wait and see attitude for the immediate future.
All the best.
