Plus there really not marketed outside of China (Panda excepted of course) and information on them is sketchy unless you read Chinese. Worst thing for me are the crap images they issue for the coins. Baaaad. Doubly so when you see how nice many of them are in the flesh.
As noted above --- "demand and sentiment", which means popularity. It's the same reason why Justin Bieber concert tickets sell out so quickly and at such high prices. Popularity.
If Australia had over 1.3 billion people, I'm sure the Perth Mint could hike its premiums. Went to Mooloolaba fish markets the other day, we sell oysters for $16/dozen, they sell them for $17 - because they can. The pro up the road sells flake for about $13/kilo, Mooloolaba sells it for $29/kilo - because they can. Big market.
They have higher premiums from the mint. Most dealers apply roughly the same fixed dollar or percent markup to products of a given size, so higher prices usually reflect higher wholesaler costs.