I'm probably going to go with injection molding. Not trying to be too fancy, just a basic plastic KB case.
Injection molding requires tooling, as well as an injection machine.
This means you need cad and experience to make a nice mold, it's not something you will just do. Then you need to actually make said mold, and the mold size needed to make a keyboard case is in the thousands, even for a 65% and at that size you need a pretty high pressure machine. Going larger means even more money, as in tens of thousands. And that is before you even pop out a single case, make a mistake, if you're lucky they can weld the mold and just re-machine the change, otherwise you get to start over. While it is the cheapest way to make a case, it only works if you're planning on making thousands to offset the initial investment which is extremely high. This is why so many customs are machined aluminum, yes they're expensive for low volume but you control the cost down to the individual item and there's far less up front cost involved.
Basically.. (without switches or caps)
Laser cut, you can do an entire board for about $100 each
Machined, at absolute bare minimum and really cutting corners, expect $800-1000, possibly as much as $3000 by the time you're happy and then $300-600 each there after.
Injection molding you might spend $10-20k on cad and molds (or more) to get your first case but each thereafter is only $10-$20 when bought in bulk.
You only use injection molding if you have to or you're doing bulk, it makes no sense otherwise.
Honestly...
Just look on Alibaba, find a design you like, email them, most will screen print your logo onto the product for free if you order enough, it's called white labeling, it's far cheaper and easier. Will it be completely custom? No, but most of the people you give these to won't appreciate what they are anyhow. With white labeling you get a known working product and it gets shipped out fast (like 2-3 weeks) as opposed to machining or worse injection molding which can easily be a 1-2 year project by the time you have your entire order in hand.