They used to say that about crude oil.... It might not always be, or perhap brass might start being hard to come by.
Nothing expensive about exploring the concept's potential or limitations...
Brass may get harder to come by, you're right. If it does, the complicated centerfire cases will be the first to reflect that. Centerfire cases cost more to produce. I'm not saying you shouldn't develop this cartridge yourself, but I suspect that after doing so you will get a good lesson in economics, and why reinventing the wheel is such a bad idea.