About 15 years ago I got a plastic ukulele (The Fluke). It cost under $200 and sounded as good or better than any uke I'd ever heard. I also have a $5 plastic Hohner harmonica that I think sounds as good as a metal and wood one. So I'm not surprised to find out that there are other kinds of high quality musical instruments made of plastic, like these pTrumpets. They are made from ABS plastic (the same kind of plastic that Lego bricks are made from) and they are great for students because they are cheaper and more durable than brass instruments.
You can even get them plated in metal, if you insist:
