I found
this article interesting.
The USS New Jersey cost $7.3 million in her 2000 restoration prior to being berthed at Camden as a museum. At the time, the US Navy estimated that the cost of returning her to active duty was well over $100 million (according to the Washington Post), although the cost of keeping her in Naval Registry Class B Mobilization status was less then $150,000.00 per year (the same state that the USS Wisconsin is kept in - all interior spaces sealed up and the internal atmosphere kept dry and clean).
I know in 2003 Taiwan bought 4 mothballed US Kidd-class destroyers for USD $785 Million USD (they'd been mothballed for a couple of decades by then). The $785 million included the cost of de-mothballing/restoring them to active duty status, upgrades, plus spares and such, so I don't know what the actual cost of just getting them seaworthy again was.
P.S. anyone know how much it cost to restore the SS Jeremiah O'Brien or SS John W. Brown to seagoing condition?