power lines

Little bit of "bush research":
Olympic Dam to GDY site is right on 500km or 300 miles.
While the publish date is quite old, the following link gives a rough 1989 estimate of $1M per mile, probably round up to $2M/mile, with inflation minus technology improvements. Highest voltage to minimise losses. NB with outback salinity levels they may need DC rather than AC.
http://www.eia.doe.gov/cneaf/solar.renewables/renewable.energy.annual/backgrnd/table32.htm

Gives a ~$600M cost for line, so by no means trivial. But probably high enough capacity to power BHP site for 50 years without an ounce of CO2. Comes to something around 1c per kW/h to service and payback the debt. Not massive, but not trivial, providing the generation costs are not great. But nonetheless transmission costs will be a big factor...