WASHINGTON — The cost of holding migrant children who have been separated from their parents in newly created “tent cities” is $775 per person per night, according to an official at the Department of Health and Human Services — far higher than the cost of keeping children with their parents in detention centers or holding them in more permanent buildings.

The reason for the high cost, the official and several former officials told NBC News, is that the sudden urgency to bring in security, air conditioning, medical workers and other government contractors far surpasses the cost for structures that are routinely staffed.

It costs $256 per person per day to hold children in permanent HHS facilities like Casa Padre…