I bet the cost is related to the money that Disney would miss out on by having the general guest population there for that time frame. So let's say the average Friday night in mid-September has X guests and the average amount a guest pays per night to get in, eat, buy souvenirs, etc. is Y, then they would likely charge something in the XY neighborhood to rent out the place. I have no idea what these numbers are but maybe...
X = 10,000 guests
Y = $100 per guest for food, souvies, etc. (this might even be on the low side)
And as far as park tickets per day, I don't know if you really have to figure that part into the cost since WDW is already getting their park tickets paid for those who come in earlier in the day.
So my XY = $1,000,000.
I would think that the guests in attendance during the private party would still buy food, souvies which could be gravy for Disney. AND the staff needed would be markedly less than a usual day as a private party wouldn't need the help of the usual 10,000 guests - depending on the size of the party.
Well, it's clear after that little calculation that I am not an economist - lol