I can advise that approximately 60 hours were spent by Police on depositions and hearings in the district court at a cost of $76.10 per hour. This cost per hour for Police time takes into account costs incurred in investigating and prosecuting any person.
Whether this figure includes the true costs of the investigation - the hours of police time spent following up leads, the time of the prosecutors preparing the case, the cost of forensic examination of the leaflets and Selwyn's computer - is an open question. But its at least a start, and more informative than the police's initial response.
As for why this took so long, it seems the police dispatched the answer in late August last year, but it mysteriously never arrived; they were able to dig it up when I sent in a new request last month.