By itself, no; not an expensive material. But good luck taking some of it and transforming it in a useful, water-tight, container with threads.
+1
Plastic items are cheap for the very simple reason that they are molded, in mass. Spend $5000K+ on a mold, then crank out 100K-100M parts for pennies a piece. This molding process offer a number of advantages, but strength is not among them.
In the flashlight world, Delrin (
Polyoxymethylene) is formed into solid rods at the factory. Then, rather than being injected molded, its cut on CNC machines, just like aluminum and other metals. The result is some of the strongest and most precisely shaped parts possible. But the material is only slightly cheaper and cuts slightly faster than aluminum. So the result isn't much cheaper.
These capsules would be $20+ were they not extras from some massive run.