Tough one to put a $$ value on.
It wouldn't catch my attention if I was scrolling through a list of domains, but I also wouldn't be shocked if a company paid up to have it represent their business. Like any other, a name is worth what somebody will pay for it.
Obviously what I've said so far is not a great help, so what I will say is that if I had a 3d business that could make use of the RD acronym, I'd be in the 5-10k range. I've seen businesses use names that are a lot worse.
Your pool of end users might not be huge, but any short name that has meaning has to be worth a few bucks to a business, if it relates to them. Its going to be a tough sell, but if you can find an interested end user you should do well.
I recommend finding a list of registered 3d domain names and scan through it to see if the initials RD might fit with any. If you need help finding a tool that will show you registered domains, reach out and I'll find a link.
p.s. 3D Radiobiological Dosimetry is a thing (google 3drd), and you'll have much more luck with that than reaching with the Rendering & Design or Research & Development angles.
Good luck!