Very tough for GA
Rick,
I used to develop systems that used HUDs for a living. To get a HUD that works well requires a lot of "stuff", the hardware on its own will be very tricky to fit in an RV. You probably know that a HUD collimates the display at infinity - you look through the display, focus on the world beyond, and see the display in focus. To do that requires 2 heavy glass plates with a large lens and a CRT to produce the image (could probably use an LCD now), all this optical stuff is heavy (say 30lbs?) and takes a lot of real estate. Mounting it in the right place will be a challenge. Your eye also has to be in just the right place to make the most of the display, an RV panel/canopy might also be too close. Let's assume you are a resourceful homebuilder and can solve these problems.
Now you have to generate the image that will be displayed on the HUD. The processor driving the display needs to be carefully matched, and the sensors providing the data need to work with the minimum of latency to make the display useable. If the display lags reality by even a tenth of a second it will be noticeable. If the data is not refreshed on the display quickly enough it will be jerky and the usefulness will degrade. Again, all these issues are solveable, but it can take a while (read, be costly).
How much do you want to pay? I'm guessing the hardware alone with be tens of thousands, plus your share of software development costs - a similar amount? A cheap system just won't be worthwhile, it will still cost a lot and won't give you any of the benefits. As a matter of interest, most combat aircraft don't use the HUD as a primary flight instrument - its not reliable enough.
Sorry to be rather negative, but I can't see a "cheap" system providing useful performance for some time to come.
Pete