In case you have not kept up with the Maker movement the last couple of years, great strides have happened in the average guys ability to build advanced micro-manufacturing facilities for very little money versus even a few years ago.
I will note that this discussion is not all inclusive as this field changes on a sometimes weekly basis.
The ability to do advanced manufacturing with relatively small amounts of money is the dirty secret of China’s present economic success. I should know, since I have been there, I have friends in the industrial automation business who have been there, and I regularly import items from China, although my capacity to produce many of the the same items that I import is growing rapidly. The parts to make these items allow are sold in nearly every small town in China. They sell stepper motors and automation equipment at the hardware stores in China.
First off and the easiest to learn tool is a CO2 laser cutter. Good quality 40 watt laser engraver/cutters are now around $425 USD, plus shipping. They do work, and I have bought several of them. While only having a small 8×10″ inch work area, they allow me to build larger things, and to produce higher value items like rubber stamps, 3D printers, key chains, signage, logo plates and even other CNC machines.
CNC routers and milling machines have come down substantially in price. I have a few routers and mini mills. A fairly decent small mill can be now had for $800 and a good large router like the Mostly Printed CNC are as little as $500 in kit form. (Files are available on on Thingiverse or it is available as a kit for $500 from the creator of the project.) Those two machines expand your manufacturing capability to be able to make other more advanced items and other machines.
The Mostly Printed CNC mill can even be used as a CNC plasma cutter. For basically about $1,000 you can now have a table mill that cost $10,000 in 2006.
3D printers have become ubiquitous. I picked up a very good Delta 3D printer, delivered as a kit for just $280. Being in the business of making printers myself, I was blown away by the quality that I got from this inexpensive kit. This tech has advanced immensely in the last year with machine reliability becoming very good. I should mention that when I first started 3D printing, I nearly gave up on it because it was so difficult to get it to work right with the print heads and other issues of the older-generation equipment. But everything has changed with the latest generation 3D printers.
Injection molding machines are also now affordable. If you have the 3D printer and/or mill, then you can builda injection molder yourself or purchase a production model from someone like Many-Maker. Your costs are under $1,000 for this. I have built a manually operated injection molder for less than $100. This allows you to basically take advantage of the free money that is strewn in the streets and trash. (Namely, plastic.) I can purchase used plastic for around 12 cents a pound or collect it for free.
Next, consider the CNC Lathe. Used EMCO brand lathes are presently available for between $300 and $2,500 USD. I am currently retrofitting one with more modern electronics and the total cost should be somewhere around $1,000. Open builds has a 4-axis CNC design that will give you nearly the same results for about the same costs.
Casting metal is another inexpensive option. You can now safely melt and cast metal including aluminum in small amounts by the use of special crucibles that work in a microwave oven. These special crucibles absorb the energy and transfer it to the metal. You can also build small gas-fired or AC electric crucibles with plans found all over the Internet. [JWR Adds: Glass fusing crucibles for microwave ovens are also now available. Find a used microwave oven for your workshoop. And never use that one for cooking!]
The investment of $10,000 can now get you the capacity of a shop that would have required the investment of several hundred thousand dollars only 10 years ago. Most of these machines do not use much electricity and they can easily be set up in a one car garage work space.
Bottom line: Your cost barrier to making things for a living, and selling them locally or online is now extremely low.
JWR Adds: I see the profusion of new home manufacturing technologies as part of the Internet-enabled Second Industrial Revolution. The advent of these many low-cost machines is something akin to when home treadle sewing machines first became widely available in the late 1800s. Suddenly, almost everyone could afford one, and it revolutionized cottage industry.
One manufacturing technology that M.B. did not mention was the advent of home vacuum forming of sheet plastic. A simple-to-construct vacuum box with an electric heating element and the hose from a Shop Vac can be used to turn ABS and other styrene sheet plastic into an amazing variety of shapes. Making the wrap-around molds takes patience and skill, but this is a proven technology. May the Maker Force be with you.