Distribution alone can easily represent more than 50% of the total markup accruing to the distribution channels vs. the original manufacturer.
Yeah, and this leaves 10% of
profit to be very little.
Sell it on Amazon? Small seller can easily pay more than 50% of the gross to Amazon by the time the fees roll in.
Internet and social media marketing/advertising? Sure, they just want 25-30% of your gross for sales they get referred to your site. Hopefully, you have created your own webstore what doesn't take 25-50% at the POS.
This is why marketers and distributors (and shows like Sharktank) like to get you hyped about gross sales. That number looks way, wayyyy better than net profit.
Does anyone know of companies which can take prototype designs to full production and sales?
There was a site called GadgetGangster that did exactly this. They sold your design as kit and/or finished product. They bought all the stuff. Carried all the inventory. Did the manufacturing. They (hopefully) cut you the check for your share every somesuch time period. It was a bit like Sparkfun or Adafruit, but with more Propeller less Arduino. I think the owner was into Propeller, himself, and probably a lot of those were his own designs.
I am pretty sure they went out of business. But if you think it's a good idea, have at it. Even one or two gadgets is a giant handful of work. Don't forget the testing, the failure/repairs and service/support.