One thing which has come to mind, and this might also be something which should be a Sticky in various categories, is the real/true total cost to raise, train and sustain capabilities, and the impact on a force structure and force structure planning by suggesting a change in kit, or introducing new kit, or even 'just' increasing numbers of existing kit already in service.
Managing a force structure a complicated business, because there are so many different components which are interdependent.
For example, if additional P-8A Poseidon's are ordered for the RAAF, then in addition to the RAAF needing to have additional air crews sufficient to permit the extra aircraft to be used operationally, the RAAF would also need additional ground support/crews to maintain the extra aircraft, there would also likely need to be more hangar space to house the extra Poseidon's, and the spare/replacement parts stocks would likely need to be increased at least a little. All of these things would require extra funding, above and beyond whatever the flyaway costs for the Poseidon's were, and some of these things would also require additional time as aircraft commanders take time to develop, as do skilled station operators for radar, sonar/sonobuoys, etc.
It does seem as though people often lose sight of the 'extras' which are also required to actually get capabilities from various bits of kit.
This. This. This. This.
@Todjaeger nails it.
To give a rough idea for those who aren't familiar with the ROM costings (these are reasonable estimations - but not exact), here are some guides (in constant):
acquisition cost is about equal to a decade of sustainment cost.
Assume $250 k / ADF member and $150 k for APS for workforce.
Weapons / stores will cost ~ 1% of acquisition. Note this ignores complexity, but it models a handful of super complex missiles for a fighter fleet or a bunch of dumb 155 mm shells for an arty piece.
Mid-life upgrade / LOTE / CAP will be about 50% of acquisition every decade. Of course, this is the constant figure, and allowing for our inflation rate and all that, the actual bill at the end of this activity will pretty much be the same figure as the bill at the end of acquisition. So your 2020 platform will cost ~$10 m in 2020 dollars and ~$10 m in 2030 dollars for its upgrade). Cyber / comms / EW heavy? Double the $ or halve the years.
The risk of overspend for anything new probably adds 30 - 50% (new in tech, not just cutting edge)
Electronic heavy? You are risking ~ 12 - 30 months late with ~ 15% acquisition bill in consequential overspending
For a worked example. Let's buy some fighters (it is a RAAF thread...) for $100 m each and there is 12 platforms in the fleet. FOC is 2025. You need to budget $1.2 b for the first purchase, then $2.4 b for the 20 year life. So our fleet of 12 that needs about 350 people in the unit has about $87.5 m per year in salaries and the like. Now, we may have to pay those people anyhow, but your 12 platform Squadron (costing $3.6 b already) just had another $1.75 b added to the bill. Shooty stuff will add another $250 m. Your CAP at 2035 adds $600 m. Our fighter squadron (of cheap, $100 m airframes) will actually cost us around $6 b.
Of that $6 b - you have to put $4.25 b into the IIP (let DPG handle workforce). All of a sudden, a handful of $100 m jets is bloody expensive.
Applying this to Loyal Wingman - a $5 mil / airframe. Each fighter Sqn needs some + training fleet + attrition fleet. So a 150 airframe purchase. No CAP - we'll replace them after a decade. They are brand new, so need their own workforce. Total bill for this cheap and cheerful UAV? $3.2 billion dollars.
Finally - be careful of assumptions. Large fleets of cheap stuff can cost as much as small fleets of expensive stuff. Take Army for example. Generally, the most expensive platforms to sustain are the helicopters - somewhere between 20% - 45% of the annual Army budget depending on what figures you use. Two years ago, the one single item that cost the most wasn't a helicopter - they were #2 onwards. It was something much more common (fleet size in the tens of thousands) and chaper (figure ~4 figures / platform). But because of that + a combination of fleet work, it ended up costing more than the helicopters.....