If that's the case, then maybe there should be a two-tier system, the public can know how much the department spends of their money, that's relevant enough, but the exact details of those expenses would not be in the public interest, and, as Fubar points out, could actually be in contradiction to it, and would be better dealt with internally.
The problem is that the police are carrying the can for previous corruption, if you had a department that had already taken a lot of flak for fiddling expenses/corruption etc, then it would be as much a question of regaining public confidence as ensuring the best use of resources.