Gurgaon stamp duty calculation: what's the real deal?

Two years ago I'd have laughed at these prices. Fed up of renting, finally taking the plunge in Gurgaon. Looking at 2BHK near Sarhol, Imperia. But yaar, stamp duty and registration cost is so high! Builders don't clarify calculation. Is it on market value or agreement value? Can we negotiate this? Any hidden charges?