They really aren't worth anything. My uni got a boneyard 737 to display near the Aero building here in India. Plane itself was free, transporting it to uni costed $50,000.
They really haven't. Routes that used to be flown by 747 are now flown by other widebodies: 777, 787, A330, A350. The 737 has neither the range nor the capacity to take over most routes the 747 used to fly.
For the curious, heres the plane : https://www.google.com/maps/@12.7994926,80.2283484,166m/data...