Prototypes are designed to answer just a few questions, so they are much cheaper and faster to develop than applications that go into production.
Prototyping is a learning experience. Its value lies not in the code produced, but in the lessons learned. That's really the point of prototyping.
TIP 16: Prototype to Learn
How NOT to Use Prototypes
Before you embark on any code-based prototyping, make sure that everyone understands that you are writing disposable code. Prototypes can be deceptively attractive to people who don't know that they are just prototypes. You must make it very clear that this code is disposable, incomplete, and unable to be completed.