Don't put the cart before the horse!

Typical market cost of a manufactured product is 4x, maybe even 5x the manufacturing cost. You have costs like documentation (the big ulgly part that has to come first), NRE (that's the part you'll like, the nonrecurring engineering), manufacturing, testing (another big ugly part, in which you verify that you've produced what the documentation says it is), packaging (you have to store and ship 'em in SOMETHING), inventory (where'll you keep the unsold units, pre-assembly parts, paperwork, packaging?), and marketing (How're you going to get rid of those unsold units?), among others.

It's likely that the documentation will stop you cold, and that's appropriate, and it's a daunting task, because if you can't develop a proper document set, you've no business trying to produce a product for someone else's use.

RE