Prototypes go through stages. The first stage is usually to establish the basic shape or design of the concept. Each subsequent phase moves towards perfecting it. At first, you will do the testing followed by friends or business associates. 
Later on, it's tested by consumers.
After thorough testing, the final stage moves into testing working production models. No new product will be officially ready for a market launch until this is done. Almost always some last minute, refinements will be required.
If your concept is costly to develop like machinery or an emerging technology, it may be difficult to prototype. In such a case, computer generated concept art may be used along with CAD drawings. Frequently new machinery is considered a prototype of sorts.
Whatever you are prototyping, an experienced partner be required during the final stages. Its R&D and engineering departments will substantially shorten development time.
Who pays for prototypes?
It depends on several factors. Initially, it's common for you to make or pay for your first-stage prototypes, which is relatively inexpensive. The first crude prototypes you make may be from any number of easy to find materials such as balsa wood or plastic blocks. By using saws, files and sandpaper, the basic concept design can be formed. It usually doesn't matter if the first designs work. The intention is to get an idea of how the innovation looks...it's basic size and shape.
Once the development process metamorphoses into making final prototypical products (working production models) for consumer testing, your manufacturing and marketing experts (or internal departments) should be doing the prototyping at its expense, not yours. Your chief responsibility is to oversee testing to ensure performance. As the inventor, it is your responsibility to address last minute changes.
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