The Rezko lot wasn't accessible from the public street. However, it abuts Obama's property and provided a private preserve that adds to its ambience. Obama bought the Rezko property for $104,500 -- $520,500 less than Rezko's wife paid for it.
This is an error. Apparently Obama only bought 1/6 of Rezco's adjoining property. That is why he only paid 1/6 of the price. The overall deal was a conflict of interest but you could say that he paid a fair price for the extra strip of land. Of course a seller not beholden to Obama probably would have charged more, but not 6 times as much.