Like any good valuation, they've taken various aspects of the building into consideration, noting that it includes a training area as well as a throne room, and worked out the square footage, the location, and an appropriate price per square foot.
In order to assess the area of the hold, they estimated the shoulder width of a female orc, and based measurements off that, giving a total area of 10,261sqft. They then looked for a real-world equivalent of Grommash Hold, and decided upon Petra, in Jordan. Alas, being an ancient monastery, Petra lacks saleable homes, so they took an average value of $328psqft from nearby Amman, and applied that to Grommash Hold, giving a value of $3,265,608.
Now, as someone who worked in commercial Real Estate for 7 years, I feel Movoto might have missed some due diligence. There is a bit of additional information about the future of the area, and it seems likely that it will be subject to some major political upheaval in the coming months. This is likely to impact the $/sqft value, and, one would think, put it below that of Stormwind!