Making a small batch would certainly help you figure out the cost/profit portion, but as mentioned, I think a lot of people don't really look at the "hidden" cost (your time, plus anything that might crop up like tools, etc) and that kind of sneaks up on them when they start production.
Yes, it's a $100 piece, but if your time is worth $20/hour, (just making that price up, it's really on you, how much is your time worth?) and each piece takes an hour or two to finish, X dollars for raw material, any tool or energy costs added, plus another hour or two for taking the order, fielding questions, (I know vendors must get tons of email, and that's part of ebing a good business, being responsive) getting the piece boxed up, dealing with the payment, and getting the item to USPS, in the above situation you are making at worst $20/piece, at best $40.
I think using what you make at work as a benchmark for what your off-time is worth is pretty fair. If you are working outside of the office, you are still working, so IF you were able to simply stay at the office those extra hours making normal pay, instead of working on your own "business", what would your income be?
This is where you see a lot of people complaining about the price of products that are in a niche market. Yes, the materials only cost X dollars, but the time an individual has to invest in getting that part from raw material to your door step has to be worth something, or they wouldn't be selling a product. And not saying you should, but double the cost of the shackle flip. Is that easier to justify the potential risk and hassle? I suspect you'll find that most of the successful businesses on here have a pretty nice profit margin on their pieces, but once all the costs that are transparent to the consumer are factored in, it's not as large as it would appear on the surface.
Maybe most people can't justify their bought not built this way, but most people can make anything within reason, and building things is fun, but if I am on a timeline, (and there is ALWAYS something else I can be doing) and my time is valuable, is the product good enough that I couldn't build it better, and can't make it for substantially less money?