I was recently participating in a summer class that involved taking the subway train to field trips for about 20-40 minutes, which I have spent most of the time folding. One of my classmates noticed this, and challenged me to fold him a helicopter. It took me about 30 minutes to design and fold (but only 20 minutes to fold another model at home) and when I finished, I presented it to him and negotiated a deal.
![Image](https://thumb.ibb.co/cRAuS5/helicopter.jpg)
The model was folded from a 23cm tracing paper, which costs about 10 cents per sheet.
Based on the model, the paper, and the time, how much do you think I should have charged him? (USD)