Casting need not be expensive, nor too difficult, although I suspect the method I use might not be durable enough for some of the finer unsupported components like the rifle in the picture you've posted.

Using this guide on Dakka https://www.dakkadakka.com/wiki/en/Simple%20Two-Piece%20Mold%20for%20Casting%20with%20Resin I have successfully made duplicate spacemarine shoulderpads. Proof picture > https://i.imgur.com/xrvy0dK.jpg (I'm going to make my own shoulderpad emblems which are chapter and squad specific by using a blank pad and carving in or adding more surface detail)

The method I used was largely similar in almost every step however I used Blue Stuff (Oyamaru) http://www.greenstuffworld.com/en/reusable-blue-stuff/8-blue-stuff-mold-8-bars.html (see also https://www.amazon.co.uk/Oyumaru-Transparent-Moulding-Clay-12/dp/B00GNT6CFY which is the same thing but clear) along with Acrylic Resin http://www.greenstuffworld.com/en/mold-making/718-acrylic-resin.html which is a porcelain like material when cast. I did not find Mould release was necessary, just wait for one part of the mould to thoroughly set and cool before making part two.

The advantage of this method is that you do not need expensive equipment, the Bluestuff/Oyamaru moulds can be remelted and reused if you make a mistake creating your mould (and you will to begin with!) and the casting resin is pennies per cast. Additionally the resulting cast piece only takes about an hour in a two part mould to get firm enough to remove. An open one part mould can be demoulded in around 30 minutes, because evaporation is quicker.

The resulting casting is ceramic hard and can be drilled and sanded, although cleanup is marginally more involved because flash must be sanded off (I made the mistake of using the mouldline remover and cracked an earlier attempt!).

The blue stuff/oyamaru is good for undercuts if you don't make the mould too thick and therefore more flexible, although you with need to remake it sooner. If you have been careful to avoid undercuts you can make the mould thicker which makes it much more durable. I chose the later method and the mould is still fine after 10 castings.

Whether this method is suitable for casting the plasticky type resins, I do not know. I don't like forgeworld style resin so I have avoided it.

But I can say for certain that at around £20 for the blue stuff and the acrylic resin material, I can now cast most parts I need to modify my models and need only make a greenstuff/carved master once.

Edit: One more thing, the youtube tutorials for bluestuff/oyamaru are universally rubbish and wrong. It was only when I used the dakka casting box tutorial and decided on pourable acrylic resin did I get anything approaching acceptable results. There is too much shrinkage and distortion pressing materials like Milliput or green stuff into the moulds.