In my theory, it would probably cost the same as buying an expensive car as it would to build a space fighter, given that the national-state of the world became a global-state, and put aside petty squabbling and humanity united to turn its eyes towards the stars.
Think about it. How much do you suppose the world, as a w(hole), spends on military a year? all that metal, logistics, and funds put to waste making war against fellow man, when it could be put into science.
If there was no war, we'd have the moon settled in 5 years, and mars settled in 15.
So how's life in your wonderful, war-free utopia? Been conquered lately? You will be. How are you going to incorporate these nation-states into your world state without killing people? There are millions and millions of people who will gladly kill you to keep ahold of their nation-states.
Society is defined by Uses and Thems, and the only way to have a world-state is to make all of humanity an Us, which then gives you the need for a Them.
Unless you have some of these nearby to use for whipping up outward aggression and paranoia, your world-state won't last long:

Also, your theory is very ill-informed. Even civilian aircraft costs many millions of dollars. An expensive car is only around $70,000. Military aircraft are even more expensive due to their more advanced technologies, materials, and design (Cessna and Gulfstream don't sell F-22s). All those military toys are very, very expensive.
Also, the costs of those 100 Ursas might actually be more than the Orion. Hell, four B-2 Spirits are the same price as a Nimitz-class aircraft carrier.