The RAAF do actually use Prepar3d for F-35 training (given it's also made by Lockheed Martin) - not the actual sim but they have some setups in the squadron offices as part task trainers. I think their version is supplied direct from LM so is not publicly available, but even still I'm guessing wouldn't be classified.
It could well be classified. It's well known that there are a couple of other sims out there, that are not available to the public.
How much do the aircraft cost on DCS? I've got some of the PMDG Boeings on MSFS2020 & P3D, they're not cheap - around US$150-200 each including the sub variants.
Then you have all the airport scenery add ons, airrac data subscriptions (Navigraph) - it's an expensive hobby.
Aircraft vary in price. The FC3 pack comes with about six aircraft (not super highly modeled, but often used as the basis for external add ons). It was $50-60. There are discounts and sales though. Holiday seasons there's often 50% off. And the Growling Sidewinder site often has 10% discount codes, with can be added to any other discounts. The game comes with the Caucasus map, though there are quite a few others.
The prices are all here ($US)
Planes
Click on buy, and it shows the price. The trials are very useful too, as they are the complete module. First time buyers can get a 50% across the board. This is especially useful when there's a sale on, as the discounts can be on top of each other...but sadly only works once.
The first jet I downloaded was the A-4. It's a fully fledged module, but is free.
Flying on the servers is free, though I do chip in a bit to GS's server. There is no pressure to do so though.
The computer was built specifically for this game (I use Macs for everything else). Any half decent gaming machine will do though. I'll look at a new GPU at some point, though the hand me down 2080Ti is okay at the moment, even if it's running at 100%.