The widely available figures for this contract (google is your friend here) say that when it started it was going to cost the PSNI £750K pa for 18/365 (excluding the cost of the aircraft). As I believe it has now gone to 24/365 and we assume a 30% increase in costs then it stands at £1m a year. Good value when you consider that in the last year it:
Attended 1268 incidents, found 28 people directly and helped groundcrews find another 33. It helped find 8 people who were missing or vulnerable and located property to the total value of £163K - oh and it also was used 4 times as a medevac.
Source
http://www.psni.police.uk/cc_annual_report_2008.pdf