I seem to recall reading a few years ago that people seeking to purchase a time expired 747-100 for a hostel and fly it to it's last location mentioned they could acquire it for it's then scrap value of US$5 million.
Still a fair bit of money. I guess the value is in the parts which could be reused and sold to airlines still operating the aircraft.
Here's a pax 744 on the market now for $27m so it's around that plus conversion cost
Boeing 747 - 400 for sale by Atlas Aviation from 1988