It's 9.5 square miles. 6080 acres. Even in California scrub land isn't going to cost $10,000 an acre, so the land costs of the US project shouldn't exceed $100 million.

(for example, here's a much larger area in the same county for $35 million: http://www.landwatch.com/San-Luis-Obispo-County-California-F... )

edit: tack on another couple hundred million for power line easements and you still haven't explained all that much of the cost difference. And I really doubt that they spent anywhere near $300 million on land and easements.

Also falling prices for photovoltaics probably, the topaz farm was completed two years ago.

Extrapolating a few graphs I found via a quick google search, you don't get a 5x drop in costs due to cheaper solar panels.

http://solarcellcentral.com/images/module_prices.jpg http://www.data360.org/temp/dsg605_500_350.jpg http://costofsolar.com/management/uploads/2013/12/solar-pv-c...

In 2015 in the US, installation labor accounted for only 11% of the cost for utility scale fixed tilt PV farms like this one ($0.19 out of $1.77 per watt project costs): http://www.nrel.gov/docs/fy15osti/64746.pdf

Land costs were less than 2% ($0.03 per watt).