So what sort of investment is-to-be expected to get into the 3D Printing game?
Filament printing--where the filament is melted into a layer and built up layer by layer is the easiest/least expensive entry point. The Printers run from $150 to $300 (the more expensive ones usually are due to being able to print larger volumes--e.g. bigger parts).
Printing speed is related to just how much material you are "melting" into the form made. If you printed a 3 x 2 x 1 block it would take a lot of material to form it solid. This is where "slicing" comes in. The slicer program will hold a given thickness for a volume shape, and then uses a honeycomb as a void filler, which saves on material. And speeds up the print.
The slicer also handles "supports" which are required where under-cuts are present, and this can be critical for intentionally hollow bits. The thermoplastic is molten when it comes out of the extruder, so it "wants" to sit on "something" to be formed. That surface uses "supports," very thin extrusions, which you then have to cut off the part.
The funky part is that, often, you want to cast hollow things not quite vertical--but that varies with the machine.
Getting 3d modeling software is a topic unto itself.
Now, there is another option in 3d printing, which is in home resin printing (it's a form of stereolithography). Liquid resin is put into a tub, and is set using UV LEDs. Initially, these are set to a baseplate which the printer moves up one printing layer increment. The resin printers only have the one moving part is the baseplate, vertically, as the LEDs are in a grid which activates per the grid.
The resin machines run from $250-300 to $600-800; and, again, size of the modeled part drives the price.
Down side to resin printers is that you need to wash the "finished" part in isopropyl, then put under UV lamps to fully set the finished part. Which is easier if you purchase dedicated equipment to wash & cure parts, which can run another couple of hundred.
You need a 30 x 60 desktop, generally, to stand all this gear up, and it wants a dedicated place to sit. This as prints can take hours, 18 and more for large parts, to print. UV resin printers have to be kept out of sunlight, as does all the stored resin and the like.
I have a bias about having a dedicated computer connected by USB, as I can check on the computer remotely. But, a thumb drive will work just as well, and does not tie up a computer (or tether it to the printer).
Fun thread, just for giggles. I’m starting to print 3D gun related stuff. I will take pics early in the process and y’all guess what it is. If anybody else is printing stuff feel free to jump in. 
Print finished in the night. Apparently the bed wasn’t as level as I thought. The tail that goes over the charging handle was thin on top and ripped out when I popped the upper loose. The threads for the barrel nut are a bit wonky too. Overall I’m giving this print a C minus. It looks decent and can be made to look better. Will be good for mock-up purposes.
