It probably depends on the product. I once met an inventor whose business consisted of coming up with ideas for inexpensive products – primarily homewares and kitchen utensils made of plastic, for example – and then making prototypes to test, molds for production, test marketing, and then selling the whole thing – idea, prototype, marketing results and production molds – as a going business. Then he’d move onto the next idea in his head.
I met him about 30 years ago, at which time he was already a millionaire and just doing what he was doing for fun. One of the products he had in development at the time was a 3-pack of hand-size fire extinguishers for kitchen use. The idea was that a cook who had a small grease fire, for example, wouldn’t hesitate to use an extinguisher that was non-toxic, wouldn’t spread the fire or make a huge mess, was right there on the counter (because it was so small, and because there were several of them in various handy places) – and only cost about 50ยข to $1.00 to use.
If your idea is for a replacement for the Brooklyn Bridge, though, you’d have to come up with a different idea to develop that.