I'm spitballing here... but what about using PCU limits as currency? If you're just starting out in the world, your PCU limit isn't of much value but getting valuable refined ingots and components is. If you're established, refining mass amounts of ore is easy but you're limited i the amount of PCUs you can build. I haven't completely explored it... but you have two classes of people who have inverse situations in supply vs. demand. There might be something there.