Prediction markets don't necessarily have to be real currency and it's a misconception that they are only accurate if you're using real money. We have run a prediction market using fantasy currency for years: http://home.inklingmarkets.com. We also run run private markets for companies...
Of course they don't have to use real money. But if you're happy with playmoney, nothing stops you from, say, just writing down predictions and checking how they ultimately do. (I do just this; great practice and useful in many ways.)
However, we shouldn't forget that many people aren't going to contribute their time and expertise without a concrete reward. Nor do I blame them.
You don't specify when you are sampling the market. It often wouldn't be surprising, or useful, that a market correctly predicts an event the day before it occurs.
