I would say that the cost of returning the SSUS to service might actually be more than the cost of new construction.
For starters, you are looking at a new power plant. Now, I don't remember the exact figures, but when the QE2 was converted from steam to diesel, the cost was quite high -- very high.
The ship will no longer be a quadruple screw ship, but will be converted to a twin screw configuration. The hull has been sitting since 1969 with little to no preventative maintenance being done to it -- at least the Queen Mary has a fresh coat of paint once in a while! Lord only knows what will have to be replaced in terms of hull plates due to negligence.
Complete engineering plans will have to be drawn up. The originals really will mean nothing at this stage, aside from being a guide. Remember that all of this also had to be done with the Vaterland/Leviathan, and again, the cost of refitting that ship -- less than ten years old -- actually exceeded the cost of new construction.
You then have the actual cost of fitting the ship out, from bow to stern. Like the France/Norway, there will be a demand for additional decks stacked onto the superstructure, and unlike the France, there is now the requirement that there be balconies galore, holes cut through decks for atriums, etc. None of this will be cheap.
If it were possible to refit the SSUS, I honestly think it would have been done before now. If it were economically justified, someone would have come up with a plan that worked.
Instead, this ship has been sitting since I was six years old. I know that I've aged, and often not for the better. How much worse for a ship that was built in 1952 for a market that would cease to exist within ten years?
Again, the SSUS was built for the punishing trade of the North Atlantic, not for cruising. The France was badly out of place in the cruising market until converted into the Norway, and she sat abandoned only three or four years before being purchased and renovated -- not 38 years.
Sorry, but the only thing the SSUS is good for right now is scrapping. There is a fortune in aluminum alone in that ship, and that, I'm sorry to say, is her destiny.