Making your own miniatures is a very rewarding experience (both the sculpting and the casting part) 
First thing you should realise is that there are multiple ways to approach this. Digital sculpts or hand sculpts? Resin casting, resin printing or metal casting? By far the easiest and cheapest option of those would be to learn 3D sculpting (or commission 3D sculpts) and buy a 3D printer. Nonetheless, many sculptors such as the Perry Brothers, Kev Adams and @Admiral prefer to sculpt by hand to this day. This process is somewhat slower and I would therefore expect the greens to be more expensive than an equivalent 3D sculpt. While you can scan ‘analogue’ sculpts to create 3D models this requires expensive laser scanners for best results (you can get really far with photogrammetry though as @MichaelX and @Reaver have recently demonstrated as part of the Brazen Bastards project).
EDIT: Forgot to add the obvious combo of commissioning 3D sculpts and casting them in metal. This can also be done and I believe this was the approach of GW from 6th edition onwards. Here you would 3D print the models first and then send them off to moulding. Definitely check with your casting partner which resins work with the temperatures during vulcanization (150-180 degrees C) beforehand! Some resins crack & become brittle or may otherwise deform.
I expect 3D printing itself to slowly push small casting companies out of business. It does, however, not scale very well for larger quantities. Casting metal still has the edge here. It’s definitely possible to cast stuff yourself but would that be commercially viable? No. In my limited experience, you have reject rates as high as 90% for more complex sculpts if you don’t feel like investing in a spin-casting machine (which start in the four figure range) – resin casting might be cheaper to get into but no experience on my end. I’m honestly more and more transitioning to the idea of using a casting service myself, simply because they are that cheap and do a much better job than I could ever do with my setup. Still useful to be able to create copies of WIP sculpts on the fly.
Check out this run-down of casting companies that @Admiral has used in the past and can recommend.
In the end, it’s really a question of what you personally like.