With a patent application on file, you have reached patent pending status and may begin marketing and licensing endeavors. Perhaps the most important time to have a prototype developed is when you begin contacting parties in industry. It is very important to have a high quality presentation package to present to potential licensees. Ultimately, every inventor would love to have the completed product as it would exist if actually in production, however, this is almost never the case. Unless you can have a prototype produced that will appear and function the way the actual product will, do not bother sending a crude one-first impressions are everything.
Computer modeling is relatively inexpensive and does by far the best job presenting a concept.
Furthermore, where additional copies of physical prototypes will cost hundreds, if not thousands, to produce, duplicates of a computer demo is as simple as copying the demonstration file to a blank disk which costs only pennies. Complexity of the invention is the greatest factor to consider before working on a prototype. For items of extreme mechanical or electrical complexity, engineering drawings are your best route. Engineering drawings give the exact dimensions and tolerances of the parts that make up your invention. They show the interrelation of parts and how they work and are connected.