Do you really know what you are talking about? 
How prototyping can benefit someone?
People may need app developers like you but they dont need prototypes
They can prototype using pen and paper
Or powerpoint
Go ahead kiddo
How prototyping can benefit someone?
People may need app developers like you but they dont need prototypes
They can prototype using pen and paper
Or powerpoint
Go ahead kiddo
you would be surprised how many new one's projects operate using the only prototype and attract investors promising new improvements
Maybe he just wants to build his own portfolio, and haven't enough ideas on what type of application to create.
If I am right, then is better not to charge anything. $20 for the app is the miserable price and I guess there is not possible to expect any unique or quality, at least it was just a prototype.
