I agree, that's a few expensive grams of plastic. But such is life. It's not the only consumer product that is priced a bit steeply (see ink cartridges, razor blades etc).
Hello, the Plantronics D100-M is the USP adapter and different headsets can be used. Therefore, the question is not easy to answer. I am using the Savi w720. These ear cups fit this headset very well.
Unfortunately, I couldn't find this information anywhere at the manufacturer's or on the web in general. With a bit of luck, someone else will answer your question.