I expect that minimum GA requirements can be met with a small box (which has a built in GPS module) and possibly a different transponder, for a total of a few thousand dollars. However, as you point out, even that could be significant to us in the low-cost-flying category.
Unfortunately these "minimal spec" solutions do not provide the user with any added benefits. To take full advantage of ADSB, you will need some way to display it, the associated WX, etc. That will tack on a lot more money.
I am of the opinion/mindset that if I am going to shell out a few thousand dollars, I should see some kind of benefit/improvement. Unfortunately that won't be the case with the minimal install. Fortunately I believe there are companies that are working on lower cost solutions for the little guy, and that is probably what I would use.
What I would like to see is a minimum-spec box, that is capable of interfacing via blue-tooth or something to an iphone or ipad to display the traffic and wx info. That way I don't have to mount expensive displays and what not. We'll see if that happens soon enough.