Looking for advice on manufacturing a physical product
onoj: For the size you are proposing, anything other than a few resistors is out of the question. Depending on the concept though, I have seen prototypes made on watch PCBs if they can work using an existing electronic design.
The trade-off is as follows - a factory/development house can make the prototype for you IF you order the final product through them in sufficient quantities. If not - custom IC/PCB design plus casing for 100 units will cost around 50 000 USD (again - this really depends on complexity and function) And that is the china price. I live there. I do this for a living. Best place to feel your way is the Hong Kong Trade Development Council or the Productivity council. I mention these so that you can get some perspective. http://www.hkpc.org/html/eng/common/index.jsp