The thing is that I can get just as good if not better ones (metal) for $7 all day long, and not have to spend time sourcing and assembling materials, and I don't even have to leave my house to do it.
When my kids were little they had a toy doctors set and the fake wooden stethoscope broke; replacing it with a real one was significantly cheaper than paying Melissa and Doug for a new one.
I'm not sure what you are buying, but it wouldn't surprise me if they were done in a factory that does medical grade quality control. While the odds are a medical problem from a non-medical grade stethoscope seem low, it still seems a couple bucks is worth the price. (if we are talking thousands I'd ask for a deeper drive into what that really gets you)
How much do you think doctor's spend on their stethoscope when they live in a developing country? My guess: About 10 USD. Think about the global demand for stethoscopes for countries where GDP per capita is below 4000 USD. It must be enormous.