Ahh, got'cha - I won't pretend to have knowledge on alloys, so shall leave that to the experts. I'd certainly like to see it in brass or copper - I know they're the most popular choices so far.
Plastics are cheap, but I expect as a novelty / marketing item it'd still be fun. If we're looking at a mass produced souvenir coin in the future I'm sure there's plenty of companies out there that can do this for us at a reasonable cost (less than £2 a coin from what I've seen.)
I know there was talk about auctioning a single, very special coin perhaps made out of titanium.. But it's sounding like it'd be too prohibitively expensive without a donation from the foundation to fund it.