Copper is very cheap, the coins would have to be huge in order to have any value compared to the cost of making them.
Have you thought about titanium or tin? (Not sure how workable these are for coin form) but they are worth about 2.5 to 3 times as much, making a smaller denomination coin easier.
Y'know, we hadn't, but that is an excellent suggestion! We'll certainly be keeping it in mind..

Sorry if I'm hijacking your thread

I was surprised at how cheap titanium was, we go through a bunch of it at work. That is relatively easy to work with. It's density is about half the weight of copper. I know I'd rather have a pocket full of USD quarter sized titanium coins that would be worth about 9.6 USD each. So you could make ones half the size for 5usd each. I bet a lot of people would jump on those for that price.