 These necessitated rigorous and costly modification programmes, thereby further increasing the type's operating costs on what were essentially low-yield routes.When the major airlines replaced their obsolete piston airliners with new Boeing 707 and Douglas DC-8 jets on their prestige long-haul routes, the unit price of second-hand DC-4s dropped to as little as £50,000 (equivalent to £ million today).The conversion of each of these airframes into car-passenger carriers cost about £80,000 (£ million today).