In the transition period the occupation forces can implement internment or concentration camps, not death camps, to catch insurgents and guerilla fighters more easily. This would both make it cheaper since you have less insurgents to worry about and you can install a competent government that can at least keep the order over time, though you'll have to do a slow withdrawl of troops as the new government is steadily given more responsibility.
Politically, this wouldn't be appealing and will face opposition, but it is effective.
Poll