Thank you for the detailed comments -- a lot of good points and questions.
Yes I think the same argument can be made for writing. When someone like Corry Doctorow posted Makers for free - http://craphound.com/makers/download/ -- He provided the world with a massive increase in value (and I know I still bought lots of copies to give as gifts). Academic articles are normally sold for $30 a read when purchased individually- so it is pretty easy to justify paying open access charges or at the very least posting pre-prints under the terms of the journal publishers contracts.
For hardware the investment model is easiest to see when seeing of the world through the eyes of an organization like the NIH or NSF. They spend millions of dollars a year on equipment for scientists - mostly locked down closed source tech. If even a small fraction of that money went to FOSH development that makes lower-cost high-quality replicable tools, the ROI would be enormous. We can make a similar argument for any government agency or professional society. That said - I think there is an ROI even for individual makers. If we use the syringe pump again. Yes it cost my lab a little extra time and money to go through the hassle of documenting and sharing the design. However, we have already been repaid -- Check out this improvement on the original pump design - http://www.appropedia.org/Lynch_open_source_syringe_pump_modifications The next time we print a battery of syringe pumps they will be better - because of others improving the design because we shared. I think a lot of makers think this way - and part of the reason why FOSH does so well on crowd funding sites.
I like your idea of a FOSH stock market - I need to think more about how that would play out and how you would set it up -- also lots of work done on an OS dividend that might be useful. Sensorica is also doing some really interesting work on trying to nail down an open value network.
Lastly, I think the vast majority of makers that are posting designs are posting their own - not copies of patented products. The patent system (which was the original open source -- after a 20 year monopoly) appears to be damaged to the point of being almost useless for this purpose. 20 years at today's rate of technological evolution is a sad joke. Patents have become lawyer documents instead of engineering documents. I know we never go to patents when we try to solve technical problems - only the literature -- and the same seems to be true in corporate labs (at many places they are even warned not to look at the patent literature as a precaution against patent lawsuits in the future -- think about how crazy that is!).