It's tough to make a living as a contract manufacturer, I think. Someone can always do it cheaper. Someone can always do it faster. Someone can always do it better. Someone can always give better service. Combining all those things in the right proportions - good enough, fast enough, cheap enough, with enough customer service - so that one can make a living is very, very hard.
Don't be wedded to any particular form of the business, but let your customers guide you (at least to some extent). Levi Strauss thought he was going to make it rich selling tents. Henry Ford didn't succeed until his 3rd car business. Etc. Etc.
If you can find a way to structure things so that your customers aren't just paying for machine time, you'll be ahead (of course). :-)
I'm reminded of a store at a mall that was selling polished cubes of glass that had laser-etched 3D images inside. Let's see... Like this - http://www.3dlasergifts.com/ The materials aren't expensive, the process must be fairly quick. So as long as one can find customers, it should be fairly lucrative.
I dunno... Maybe little 3D-printed charms of some sort for motorcycle enthusiasts? http://i.materialise.com/materials/gold
Best of luck!
Cheers,
Scott.
(Who realizes he too often does little more than state the obvious here.)