A CH-47 also
reportedly costs five times the C-27J to operate per hour, for what I would argue is far less utility to the RNZAF.
How much? Well it is estimated that the US fleet costs over US$3000 per hour, and it is estimated that the RAF fleet has costs more like US$8000 per hour. How much would a fleet of 4 or less CH-47 cost NZ to operate?
Regarding the C-27J/C-295, the cost differences are not insubstantial either. The C-27J costs something in the order of US$33m each and the C-295 about US$22m each (a C-130J is about US$80m). It is generally agreed that the C-27J operating costs are also significantly higher, with EADS (not exactly independent, I know) estimating that the C-27J could cost up 50% more to run per hour. Others have said more like a third. 33-50% is quite a chunk of change.
The "commonality" with the C-130J is blown way out of proportion, as the only commonality is the engines and some avionics. Pilots and aircrew would still need to do the full conversion course as the C-27J/C-130J are so different, and the only group of people to benefit would be engine maintenance personnel who could perhaps work on both types.
On the other hand, given the PW100-family of engines are used far more widely on some very common commuter aircraft (ATR-family, Dash-8-family) it is going to be far easier and cheaper to find parts and maintenance people for than the AE2100 which is just used on the C-130J.
Further, how certain is it that the RNZAF is going to choose the C-130J? If another type is chosen then the commonality issue is pointless, and you end up with an aircraft that was more expensive to buy and is now more expensive to run.