Here's my take on how to build a really low cost aircraft and win the coveted award for "cheapest RV".
1. Extensively use second hand componenty. The only downside being that on getting airborne you move straight from the expensive build stage to the expensive repair stage.
2. Don't bother painting the aircraft. Just pretend it's completed when in fact it's not.
3. Refuse to add the cost of all tools needed to fabricate (and eventually maintain) the aircraft into the final summation (that alone will save a bundle!).
4. Keep no accurate records of the cost of all actual purchases and just guess a figure at the end of the build. The final figure will in all likelihood be way under but, hey, no-one's checking on you.
5. Leave out all of the basic safety equipment that one would expect to find in a certificated aircraft (with luck you'll probably never need that Halon fire extinguisher anyway).
6. Assume that anything that is not riveted to the aircraft is not a real building cost (that'll get rid of building insurance, transportation costs to an airfield, and hangar rental for final assembly....to name but a few possible items).
7. Quote your build cost in 1980s dollars (big savings there!).
8. Under no circumstances replace componentry damaged during the fabrication process. Just rigidly adopt the popular "bog it up and build on" philosophy.
9. And if all of the above steps fail to produce the savings required, just convince yourself that the final cost of building the aircraft is the fictitious figure you've been spinning to your wife (has the additional benefit of alleviating guilt) .
Readers of this thread may be able to think of other neat ways to "reduce" actual build costs.
