I think I would like a second resource for trading. I can imagine green trading ships that spawn blocks on payment pretty easily. Ram them to pay them and they spit out a block on the other side.
Making it depend on fuel wouldn't be much fun though. It would force you to spend all your time mining. Cash should be dropped by something interactive like blocks - letting you crush them for resource instead of just abandoning blocks you don't like. Or perhaps you have to hunt worms or something bigger.