Why not just buy a Piper Warrior, or a Cessna 172, or a Cessna Citation, or some other conventional aircraft? The thing will already be built, or in various stages of decay, and we could be flying tomorrow. A freshly completed RV is going to put a debit in your saving to the tune of over $35,000 and set your retirement back a few decades, so the reason for building is not a matter of saving money....or is it?
With your $35K Warrior or C172, you get to enjoy the wonderful eccentricities of a twenty-five year-old airframe with instruments and avionics of various and sometimes unidentifiable pedigrees, not to mention the wonderful cabin aroma of unnumbered sweaty students and their meagerly paid and overworked instructors. You also get to adopt an A&P mechanic that will consume more disposable income than your two teenage kids. Don't ask me how I learned all this, just ask my kids. They are the unwashed street urchins that used to have a nice home before their Dad got involved in the "hobby" of aviation.
After building your RV, not only will you have a fresh, new airplane, you will be able to receive the Repairman's Certificate that will enable you to perform most of the work necessary for the proper upkeep of your new toy. You also are free to experiment with novel or unusual systems and powerplants if you choose. I think this is why this type of aircraft receives what is known as an "Experimental Certificate". I am considering a 350 watt sound system that will play the sound of a big round engine as I make low fly-bys at pancake breakfasts.
Seriously, the decision to build goes far beyond monetary values. It reaches out to that urge many of us have to create something wonderful from a box of ordinary matter. It is the desire to begin a project that will give us the incentive to look forward to the next little challenge in our spare time instead of wasting away in front of some mind-numbing sitcom. It is the source of pride that insures that every visitor to our home gets detoured through the shop on their way out to the car. It is the dream that someday, we will be able to experience an adventure that only a minute number of humans have enjoyed, a journey thousands of feet above the earth in a wonderful home-built machine as it swims through the atmospheric ocean.
The decision to build an aircraft is not one reached though a rational mindset. It is a decision that is made because WE HAVE TO DO IT! It makes no sense, but neither does love, children and taxes.