Do people usually get compensated for this?
YOu must be joking. A prototype is a part of a complex development process. By any means, it is paid - in full, according to development time.
What's reasonable compensation for building a product prototype?
DEFINE PRODUCT PROTOTYPE. Seriouisly. Why do people come with questions that have ZERRO information and even think they get an answer. SOme prototypes are fast - weeks. Some take a 50 man team 3 months - because they are complex. What please is the product? ;)
If so, what should I ask for and how would I come up with that number?
How are you paid?
Fixed price? Estimate hourly work, put your price on it, multiply by 2.5 to 5 for fixed price. Work by hour? Simply estimate hours.