I do not use OEM ink on our Mimaki, that being said I pay .10 per ml for ink. Changing a DX5 head in a Mimaki JV33, literally about 15 minutes, plus another 5 minutes to run a new dot position calibration. If you shop it around you can get a new DX5 head for under a grand, taken care of it will last a few years.
Of course I realize I tend to be more resourceful and adventerous than most when it comes to working on my own machine. Most will simply deal with days of downtime and wait on a tech to come replace it for them and most do not run bulk ink that comes in 2 liter boxes so my numbers/experience are not quite average.
For utter simplicity and ease I cannot knock how you replace the heads on a HP though. I dont think it gets any easier or faster than that. I also woudl readily agree that Epsons price gouging on their piezo heads is harsh but then again, grand format heads are not really cheap either. Also, yes, on older 3 and 4 head DX4 head machines, it's a good bit more complicated to replace the head with much more calibrations required. Probably much the same with other newer multiple head machines with "permanent" heads as well since there is the whole physical alignment needed between each head. Single head machines tend to eliminate much of the calibrations needed since all channels are within the same physical head.