It is possible to get into injection molding for well under $15K, but it is highly dependent on the volume and dimensions of the item you want to make, and the quantities. And your skill.

Injection molding machines get big and expensive quickly. But there are some hobby / education molding machines that are relatively inexpensive. They'll be limited in the part size, of course, but it will be very educational. You can find used industrial machines, but ones in a just-right size are harder to find.

You can make molds using high-temperature epoxy (hundreds of shots), high-temp SLA resin (presumably at least tens to hundreds), aluminum (thousands), or steel (often refurbished for very long lives). Having a CNC mill will be very helpful--something like a Tormach 440 is $7K or so new, and is useful for all kinds of things besides mold making. Depending on the complexity of the part and the finish you need, this is probably intermediate machining skill.

Machines are often rated in clamping force (e.g, "15-ton"), but shot size/volume (usually expressed as the weight per shot in polystyrene) and injection pressure are also important. Horizontal machines can take up a lot of space, but can use gravity to clear the part after it's made.

For smaller parts, Morgan makes some nice benchtop machines, but they're pricey. If I was going to buy a new machine, I might look at this cute little horizontal: www.robotdigg.com/product/657/ It's about $9K I think.

For me, it made all the sense in the world to buy an old import vertical 15-ton machine ($2,800 on eBay) and design and make my own aluminum molds for a product I sell. I learned a lot, and now can make other parts. I spent less than the cost of having a mold company make me a single mold (I think the quotes I had were $3,500 or so for my relatively simple, small part). But I have access and experience on a CNC mill, lathe, etc. If you don't have that, or don't live near a makerspace, then you'd need to factor in the machining equipment and tools.

I currently produce an ABS case for my product, actually resembles a DB-25 shell. About 2.5 x 1.5 x .8". I did use Fusion 360 CAD and CAM, and a Tormach CNC mill. Used aluminum for the two mold halves and spacers, actual ejector pins (they're inexpensive) for the ejectors. About a 40-second cycle time. It took quite a while to get everything put together (the manual is in Chinese, and actually is for a similar but not identical machine), and I'll probably add on some microprocessor control soon to make it a bit easier to run. It is extremely pleasing to be able to sit down and fill up a box with custom plastic parts. Need to add a logo or product name to the part? Throw it on the CNC mill and you're set.

How much would I pay to take a course to learn how to create molds and set up and run an injection molding machine? Right now I've spent less than I would having an outside company make just the molds. Your outlook may be different, but learning and creating actual products (subject to some size limitations) is very possible. It all depends on what value you place on learning new skills, versus the raw economics of it.

The booklet from Morgan called "Cutting costs in short-run plastics injection molding" covers simple mold making, and the case for using high-temperature epoxy for short-run molds.

The NYC CNC folks machined their own mold from a pre-built mold blank on their CNC machine, then had a company do the injection molding.

(I'm omitting the weeks I spent stripping and rebuilding a small pneumatic machine--it was just too small for my part. Another learning.)

By the way, I had a toy injection molding machine as a kid. Small but completely functional--came with a mold for a toy soldier, I think. I'd cut up sprues from plastic models. Probably burned my fingers more than once, but an excellent experience for a kid. Wish that was more common.