How much does an inventor stand to make in the grand scheme of things?
The prototype is a piece of art which I can always make custom. The company I'm dealing with can get this thing made in China and make them more affordable to the public as a mass produced piece. What percentage of profit should I get as the the prototype creator?
Generally, the inventor takes very little of the ongoing profits because the inventor usually takes very little of the risk. I would recommend you ask for a set dollar amount per piece produced rather than a percentage of profits. You can ask for an amount up front and royalty as you go.
The only real way for an inventor to make a large amount from their invention is to keep control and build a company around the invention.