Here's how I made my first cartridge case.
First, I made two molds with tin cure silicone, the cheapest variant. Perhaps I'll make another one with platinum cure later.
The cost of the molds is about 12,= euro's in total, but they can be reused several times. Not sure how many times yet. Might vary between 20 and 120, judging by the reports I read on the internet.
It took 10-15 hours to cure until I could take the originals out of the mold. After that, I had them post-cure for two days. I was very pleased with the results.
Pouring the PU was a totally different matter. The silicone had a pot life of 40 minutes, which means I had plenty of time to mix the two components and pour them in the container, but the PU I had bought is a fast curing variant with a pot life of only 2-3 minutes. So when I started with the second mold, it had become way too syrupy. Nevertheless, I managed to get it all in the mold. I unmolded after 60 minutes, let it cure a little longer and then used a vile and sandpaper to clean it up.
IMHO it turned out not too bad for a first try. I have plenty of PU left to try a few times more. By my calculations the cost of one case, not counting the mold, is about 1,25 euro.