The cost to put a kilo in orbit will start dropping precipitously as more for-profit launchers and companies open for business. The physics are also not a problem. If you look at it from a pure physics perspective, the amount of energy required to orbit isn't anything special. In fact, if you could deliver that amount of energy any way but through a rocket, you'd be paying a few dollars.
That is true, but it doesn't change the fact that nearly everything you can mine in space you can mine on Earth. Earth is made of the same stuff, and is much closer.
They only thing that could reverse this logic is if we set up really strict environmental standards for mining on Earth that would force companies to do the mining off-world. But then, this would hugely increase the prices of raw materials and unless you have an eco-friendly way of moving millions of tons of raw materials from space back to Earth, it's not much of an improvement over terrestrial mining.
Unfortunately, rockets are the only way to do it, but their high price has as much to do with government manipulation/control of the market and lack of competition as it does with the inherent difficulty of building a rocket.
Yup. Give me Skylon or give me quick and painless death, because I can't watch it any more.
Lastly, the argument that start up costs are high and that it only makes sense to use space resources in space doesn't convince me. The price of opening a new mine on earth is high, or opening a new airport, or building a new airliner, or any other number of extremely expensive commercial operations that go ahead anyway. If there is a buck to be made someone will try it.
I once played with an idea for a sci-fi story, which demonstrates the need for off-earth colonies:
sometime around now, we 'accidentally' create a "strangelet" particle in one of our particle accelerators. It sinks to the core of the Earth and starts converting its mass to some sort of quark-gluon plasma. The rate of this transformation is exponential, and scientists calculate it will take another 6-8 years for the effects to be felt, and another year from then for the surface to become uninhabitable, and then a few weeks for Earth to be totally consumed. There is no way to stop this.
It's kept secret, of course, but major governments launch frantic efforts to send at least a few thousand people to Mars to continue the human species and preserve at least something of the Terran genetic diversity. Of course there are disagreements over what the share of people from various nations should be among the colonists, so many pull our and go alone (China most prominently). Rich individuals who have learned about this plan to set up a private retreat on the Moon in the meantime.
As the doomsday approaches, governments can no longer keep the imminent destruction of Earth secret. The planet plunges into chaos as earthquakes and volcanic eruptions start making the planet uninhabitable. The last ships to leave Earth orbit can actually see the Earth surface burning with lava from supereruptions, and later they witness the final collapse which tears Earth apart to create an accretion disk of red-hot rocky debris. The Moon is actually hit by pieces of Earth thrown out during the implosion and the makeshift shelters there are damaged or destroyed (few eventually manage to evacuate to Mars and the Moon is abandoned).
400 years later, Mars is a terraformed, living planet whose inhabitants see Earth as something of a myth (in the same way we now think about ancient civilizations which no longer exist). One guy, who is a bit of a weirdo because he is obsessed with studying Earth history, geography, culture, ecology, basically all things Earth-related, by chance finds evidence that the "strangelet" accident was not an accident at all, but a deliberate sabotage by an alien entity. He and a few other people devise a way to use the frame-dragging effect around the collapsed remnant of Earth to send someone in a small capsule back into the past. Drama ensues when the government wants to stop them from carrying out their plan, but they eventually succeed. A closed time-like curve?
Bottom line - we need human presence beyond Earth
Alright, you got me there. 
