It can cost tens of millions to certify a new private plane like the Cirrus. The more complicated the design, the more it costs to certify. That's why so few new designs have been developed in the past 20 years. Instead, several companies keep attempting to revive old designs like the Luscome Sedan, Tiger, and Rockwell 114 in place of a completely new design.

The cost to design, build, and certify a new airliner typically runs into the billions (the Boeing 777 cost over $3 billion and the new Airbus design will cost somewhere around $12 billion), although a good chunk of that is for factory space.

As for attempting to certify the White Cloud and SS1 to carry commercial passengers, I just don't see it being economically viable in this country unless the FAA comes up with some special certification category. Under the current regulations, you'd also have to certify the rocket engine and perhaps even the jet engines on the White Cloud. The WC uses engines from the T-38 jet trainer and I don't know if those engines were ever certified for civilian commercial use. The core engine might be certified but I suspect the afterburning version wasn't. Those innovative avionics in the White Cloud and SS1 would also have to be certified or replaced with certified units. In addition, under the current certification requirements, Scaled Composites would probably have to build test articles that would be tested to destruction to prove their structural strengh and durability. All of this stuff adds to the certification expense.