How is a plot of land priced?

My players want to use the Downtime system to build a commercial empire, including a major hub in the middle of the Alabaster District of Magnimar (read: "the most expensive and exclusive neighborhood in one of the biggest cities on Golarion).

I haven’t found anything in the Downtime system (or anywhere else) to help determine the price to buy land (ignoring any buildings that might be on said land).

Have I overlooked something, or is this purely in the realm of GM discretion? And, if it’s the latter, is there a set of guidelines or a starting point that has worked well in the "real world"?