How do you raise the stakes in an experiment without actually paying people money?
Make them create something with their own hands, something hard to make and use that as currency.

I first became aware of this type of experiments from a ted talk by Dan Ariely which I recommend you watch, specially the last part:

What Pushes Us To Work Hard — Even When We Don't Have To?

The gist from these behavioral economics experiments, is that we value things we make substantially higher than an impartial evaluator,even more if making them was difficult,challenging or rewarding; so if you don't have money but have willing  experimenters, you could possibly make them create something, let them assign a value and then use it as currency.

You experiment would be as follow:


Create something - Origami,painting, sculpture of themselves 
Assign Value - Ask them how much would they sell it for
Substitute for currency - Take their thing away from them, now you have something you can reward them with or even fraction.