I believe either Solidworks or Autocad has a mold making tool. Render the part and it will tell you how to make the mold.
I've read that SP charges $45 for custom keys, which means about that for the mold (they are small luckily). However, they already have most of the CAD info in hand as well as an experienced mold maker making them (injection mold makers are highly specialized), they may even own the cnc mill. And that's for one key. Get it wrong (did you account for shrinkage?) or need multiple keys and the price goes up that much, every time.
Now the worst part...
You could get them 3d printed, however, my experience is that it's MUCH more expensive to 3d print metal than it is to have it machined. Unfortunately, finding a shop willing to take a bunch of small one off work like that will not be fast or cheap. They dislike jobs like that as they tie up the machines and manpower for little profit. So they may do it (expect a few to blow you off) but expect them to fit you in when and where they can. Could be weeks or months depending on their workload.
Bottom line, unless you know someone, don't expect a full set of molds for anything under $10k, probably closer to 20 or 30k. You could cut that by 60% if you have them done in China, however a bit of warning, while they may do it cheaper, and maybe even faster, you may not get anything like what you expect and you have no recourse on getting your money back. If you have never done business with Chinese manufacturers, don't set your expectations high. They are capable of good and cheap, but quite often the language is an issue, and even if you get beyond that, there are a lot of unscrupulous manufacturers who will do everything they can to make it as cheap as possible. That includes wrong materials, improper surface finish, poor packing, wrong shipper... They don't care, they got paid, you have little to no recourse, and there is 50 more people waiting in line behind you. I've known more than one company who had to send someone over to watch over the manufacturing process before it was finally done right.
Once you have the molds though, yes, you can crank out keycaps for pennies.