For those wondering: this is here:
https://www.google.com/maps/place/42%C2%B041'08.5%22N+87%C2%...
and the land is located adjacent to the land that microsoft took over to build a huge datacenter in the previous location where Foxconn was going to build a giant LCD factory (or at least said they were going to but never did). So the value of this mostly comes from the fact that it is adjacent to an already huge chunk of land that Microsoft owns. Also likely Microsoft is getting some tax breaks for this.