The tower has actually been in operation since September 1990. So the time needed for construction was just 2 years and not 16 years. For the first years it was operating as a simple drop tower, without a catapult. The catapult was added later in 2004. Construction costs according to German Wikipedia were more than 24 million DM (12.2 million euros), with a further 4.2 million euros for the catapult. They are doing on average about 400 drops per year. The time in micro-gravity is 4.74 sec for a simple drop and 9.3 sec with the catapult. (source: Wikipedia) According to my estimations this amounts to about 18.72 hours of micro-gravity in 24 years.

That's still expensive, but not outrageously so.