To be fair, they use it quite a lot for exercise and recreation, so it's not like a completely disused open space. Other very expensive cities also have relatively huge parks (like Central Park in New York or Golden Gate Park here in San Francisco). I've heard people try to estimate the real estate value of Central Park, which may be somewhere around $1,000,000,000,000.

The size of Tempelhofer Feld is apparently almost exactly the same as that of Central Park, although it's probably much less used compared to Central Park. It looks like Central Park might get somewhere around four times as many visits.