Trailing 3 year averages would be the norm for this as BurningRoF stated.

See the pre-feasability study from 2008 for the kinds of scenarios they run. Pages 382-387 may be of interest especially. All scenarios need to be looked at including the possibility of copper going down to $1.50/lb for a lengthy period in the coming years (highly unlikely in my opinion, but one must cover all based when investing billions).

I think that a significantly higher-grade starter pit, higher 3 year avg. metal prices and a way bigger deposit that keeps growing it seems (read 40-50 year mine life or more?!!) make the economics increasingly attractive on CUU.

PFS 2008: http://www.copperfoxmetals.com/i/pdf/09152008_43-101.pdf