I'm an Electrical Engineer/Entrepreneur who has done hardware design for small startups.
It sounds like with a working Arduino prototype, you are well on your way.
How is your idea/prototype being received? Are people interested and have you
been able to get market research/feedback on what people would be willing to pay?
Redoing the circuit design for a non-Arduino second prototype and doing the mechanical casing/enclosure
is not a trivial task, but doesn't have to be exceptionally costly or time consuming, either.
Depending on what circuit you are designing and what sort of molded casing you need,
you could have a self-designed prototype in a few weeks to a few months for perhaps as little
as a few thousand dollars.
The real key in a hardware design is making sure that it is designed for easy manufacturability and low cost
when you are ready to build in quantity.
You'll probably want to do you initial runs using a local or regional contract manufacturer,
but down the line you may want to do mass production in China, etc.
Setting up this pipeline and doing quality control with this is often the most difficult part of the
hardware process (just ask the Wakemate guys), but can definitely lower the cost.
You have to balance the numbers, though. If the quantities aren't there, I've found it best
to fabricate locally.
Regardless, feel free to drop me a line if you are interested in talking more about
the process, timelines, and costs involved.
I may be able to help you out with the circuit and meechanical design, as well, if you are interested.