How do you raise the stakes in an experiment without actually paying people money? I am trying to run a psychology experiment for one of my courses and it involves people trying to guard some "treasure" of theirs and trying not to lose it.

My problem is that I can't ethically (or legally) force them to offer up their own money and then actually penalize them for not doing well.  At the same time, many of the participants that I've run so far have figured out that it's all a ruse and it's fake and they're not going to actually lose any money so the data may not accurately show how people react in those situations when there is something on the line.    Or I could give them a hypothetical scenario in which their job is tied to how well they do but again, it's not real so people often just don't try.

Ideally, I would have some sort of funding for this where I can offer them $10 if they do well and slowly start taking away money as they fail to give them some incentive.  However, I don't have any of that (it's a course project) and I'm trying to figure out how to incentivize them.  

Any help or ideas would be greatly appreciated.  Thank you!
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.