Okay, so this idea hit me the other day and it is not fully fleshed out, but I think it is a good start of something.
First, here are a few assumptions:
There is a finite amount of each material to be mined in a given system.
Systems will be depleted over time.
When a ship is destroyed or a person is killed some material is "lost" forever, while some can be salvaged/reclaimed.
The economy needs some sinks and faucets in order to prevent excessive inflation