Looks like you got some good pricing info from applewood but I want to add another cost you haven't factored in: Parts for the 3D printer.

You will need to replace nozzles and belts (or gears) and power supplies and arms and fans and all that kind of stuff if you start getting orders. Running the thing for 18 hours a day is WAY more wear and tear than running it a total of 18 hours over two weeks.

You'll get into a week or two of printing, and shit will start failing. And nothing ever fails alone, because it puts more pressure on the thing before it and after it in the chain. So those things fail quickly after you replace the first part.

Then yer gonna say "screw it!" and buy a second 3D printer, or upgrade to the more industrial one... but then little differences between the designs will start to manifest themselves, and you'll find parts made on one machine don't interlock with the parts made on the other one, and ...

... well, now you know why all* kickstarters fail: Making one is easy. Manufacturing is hard.

So add $1 to every one you sell for "future upgrades". The future is much closer than it seems...