There is not any great demand for them that I can see. What sources are you getting this information from?
I would chalk this up to the fact that most people just don't have a very good idea of what used computers are really worth. They always seem to think they are worth much more, so they try to sell them for more.
Regardless of what some people say, eBay is not a good judge of what a fair asking price is. eBay (and the like) may give a good estimate of the current going prices, but those prices are not always the fair price. Still, as long as enough people pay the higher prices then the fair price won't see the light of day.
A 1-2 year old G4 Mini should only cost no more than $350 nowadays. And that is only if the machine has the higher end specs. Stock G4 Minis from the same time frame should cost less... maybe $250. Those are the fair prices. The price should only be higher if extra peripherals are included (monitor, keyboard, mouse..etc).
Interest in Macs in general has gone up lately, so I would assume it is just greedy sellers trying to squeeze out as much as they can from a potential buyer, when the systems they are selling simply aren't worthy of the prices asked.