It depends on what you are trying to achieve with the prototype.

For example are you in the early stages of research and ideation or are you in the market testing phase?

Injection molding can be suitable for prototyping in some cases, it all depends on volumes though. If you want to test market feedback you would most likely need a large number of products to get sufficient data. Some companies provide cheaper molding solutions specifically suited to prototyping and achieve this by making use of aluminum molds instead of polished and plated tool steel.