It was notable when, in 2014, a D.C. office building — PNC Place — first traded for more than $1,000 per square foot. So what can we say about $1 million an acre? In Loudoun County, in recent months, that’s become the standard — at least for land on data center alley. Investors that dropped in only a few years ago are cashing out in a big way. The latest parcel to trade at that threshold is a 39-acre tract in Sterling, on the west side of state Route 606 across from the auto auction.