Companies that do this sort of thing do so on the prospect that it will lead to larger orders, not all, but most shops REALLY prefer large runs, and as such will charge you a lot and often you get stuck waiting for a slow down on high volume runs.
Where it gets really tricky though, is if you have never done one before, even if you do know CAD, getting even something this simple right the first time is slim to none. It may work, but odds of it being right are pretty slim. CAD takes time to really get dialed and even once you get good with it, all it takes is one spot where you forget your tolerances (you know the tolerances of the machine making it right? How about minimum radius?).
More than this though, just because you can design, doesn't mean it will be practical. What I mean is, you can design something and it works perfectly, but a slight change could cut the cost to manufacture it in half. And that comes from experience (unless you get lucky). In a case such as this, that could be as simple as how the screw holes are done, since this could be done in a single milling operation, or you may have to flip it over and do a second. the moment you have to flip it, your costs just went up.
While you may think, well, it's only one... but it won't be just one.
It will be $300, and if you pay someone to help design it it may only cost $200, but you will still have to pay them the $100.
Then you will have revision 2, which will be $250 in total.
Then you will want it anodized or powder coated, which will run you $80 or so unless they slip it in with another batch of similar color, but even then it will probably be $50-60.
So yes, you can make your own case and it will run you between $350 and $700 and between 3-12 months time, or you can just buy an off the shelf 60% case for $130 and be done with it. Excessive you say? Maybe, but probably not. I'm not trying to discourage you, just make you aware of the realities of small scale production. One-off's get expensive REALLY fast, particularly if you have a specific time frame you need it in.
By the way, Sketchup, is not a good way to design something for CAD.And if you are in the US, I would advise you to do everything in standard, as we have had several shops turn our work away because they didn't know how, or didn't want to mess up the conversion from metric to standard.