To bring back a single aircraft for display purposes only could probably be done for a few 10s of millions. If and it's a big if BA or AF allow one to be used for such purpose.
As for bringing back a fleet to operate revenue service I think you'd be looking a several hundred million, and quite possibly a lot more. It would require significant commitment from the airlines, a manufacturer to sponsor the type approval of the aircraft and Rolls Royce to provide support for the engines. In short it would be a pipe dream. Despite the nostalgia I think it would be difficult to get airports to agree to the aircraft operating due to the very high noise levels. And finally the fares would have to cover the reentry into service costs as well as the direct operating costs. As such they would be astonishingly high, I'd have thought Etihad's Residence would be cheaper.