Reddit mentions of Easy Wood Tools #5200 Mini Easy Rougher Replaceable Carbide Insert Cutter Lathe Wood Turning Gouge with Maple Hardwood Overall Size 16"

I recently got into turning pens so I will share my experience.

As far as a learning curve, turning a basic slimline wood pen is very beginner friendly as I think you have seen. Turning a more complex shape requires a little more skill, but even then I think you are only talking about making a few pens to get it right. After that, turning other materials (acrylic, antler, dyed woods like OP, etc.) requires more patience and possibly better tools. The actual assembly of some pens can also get trickier (laser cut kits coming to mind) but even then I think after a couple pens you can pretty much get it down.

I am by no means a master, and i don't mean to make it sound like crafting a perfectly turned and polished pen is easy, just that it isn't difficult to make something that will look good to most people.

As far as tools let's start with the necessities:

Lathe: It looks like OP has a great lathe for turning pens and /u/PenPenGuin pointed out it looks to be about $600. I have this lathe from Harbor Freight that is $200. I could probably work faster with a better lathe, and it might not last as long, but it works for me right now.

Chisels: I started off with a cheap set of HSS chisels ($20) for a lathe, got tired of sharpening them all of the time so I got a carbide tipped rougher and finisher like this ($90 each). Another thing to consider is the cost of items to sharpen your chisels or replace the blades in the case of the carbide ones.

Sandpaper: They make a box of assorted grits on rolls like these ($20). You only need a small amount per pen.

Finish: Whether you are making wood or acrylic or whatever you will want to put some type of finish on your pen to give it a great shine. Friction polish is popular for wooden pens or you could use a CA (cyanoacrylate) finish.

Wooden blanks: You could make these yourself from wood scraps although I never have. You can find starter kits that are pretty cheap. If you buy a non-special wood in bulk you probably pay less than a dollar per blank. With that said you can spend a lot more on fancier ones (dyed burls, fancy patterns, etc.).

Pen Kit: Same with blanks really. Beginner sets are cheap. Advanced sets can cost a lot more. Generally, the slimline kits are going to be the cheapest.

Assorted tools: Unless you buy pre-drilled blanks you will need to drill them with a drill bit specific to the kit you are using. Most people use either a drill press or lathe attachment for this. I guess it is technically possibly to use a drill for this, though not recommended. These tools are one-time costs that you may or may not already have. Also CA glue.

Random pen-specific stuff: pen mandrel, bushings (specific to pen kit), barrel trimmer (specific to pen barrel size). Usually you can find some type of starter kit that will include these things.

Nice to have stuff:

Pen-drilling jig/vice: Makes drilling down center easier and faster. You can make your own or buy one.

Pen Press: Makes putting together parts easier. Also possible to use a vice.

Fancy Polishing Kits: If you plan to sell your pens you will eventually want something that can put a really nice finish on your pen. Something like buffing wheels that go on the lathe.

TL;DR: You need a lathe and chisels that could cost you anywhere from $200 to $1000+. After that I would look at a pen making starter kit like this ($135) (note: for about $45 dollars more, pack 2 adds a pen press). That will get you started and you can upgrade parts if it is something you enjoy.

Like I said, I am definitely not an expert so if anyone wants to correct me or add anything please feel free.