The cost for the prison facilities in 2015 was $445 Million. That includes the prison only, which pretty much can serve no other role than as a prison, short of being torn down and rebuilt as a different type of facility. That does not include many millions more for other facilities and operations at the base. Divide that figure by 40 inmates and it amounts to well over $10 Million for inmate.
That does NOT include the cost of the Military Commissions, so in reality, the figure is substantially higher.
I would close Guantanamo Bay, repatriate most of the detainees to their home countries and any real baddies I would transfer to the United States for trial before the Article III Courts.
I would then return Guantanamo Bay to Cuba’s sovereignty, with perhaps a 1 to 2 year transition period sufficient to remove United States property from the base.