That's actually a low figure, compared to some. Events such as the Arlington and Copperstate Fly-In's, which use arrival procedures derived from the Fisk procedure that has been standard for decades at Oshkosh (and thus more controllers, due to the arrival controller function), are being quoted figures around $10,000-- per day.
It's not hard to see where those types of numbers come from: controller hours are charged at overtime rates, under the theory that a controller on TDY at a special event must be replaced at his home facility by a controller on OT. The controller average hourly rate is in the vicinity of $60 per hour, so the OT rate would be around $90 per hour, or $720 per 8 hour shift. Add on the travel and per diem charges, and the daily cost for each controller will approach $1000.
8 controllers per day would be $8000, then you have to add on the 25% administrative fee the feds are charging. Presto, $10000 for every day the tower is open.