Thread:
http://www.habitablezone.com/space/messages/469254.html

A couple posters expressed their disbelief at the possibility of moving
large numbers of people [billions of people] off Planet Earth.
Now, I would agree with the idea that "The reason" for going into
space should not be to lower the population on Earth, but instead my point was that
it could be "a reason" to do so. My point was that it would allowable or something which
was doable.
It's somewhat like saying that if humans could fly airplanes, then some day,
you could move an army, 1/2 around the world by using airplanes. Which
doesn't mean that in 1912, we should attempting to build planes so that
we could fly armies 1/2 around world.
In 1912 the military purpose of planes was more suitable for scouting enemy territory,
rather moving large numbers of personnel and it would been a very bad idea in 1912 to
attempt to make airplanes that carry large numbers of people 1/2 around the world.

And btw, in the civilian world in 1912, building and flying air planes was something that was
mostly fun to do- and you could make money by entertaining people, i.e Barn Storming.
Also for civilian use, airplanes were fast, so instead taking a boat somewhere, you
could use a sea plane. And govt supported such things commercial enterprises
as air mail services.
Even in 1914, when you had the first airline providing regular passenger services,
it would have be foolish to be trying to use 1914 airplane technology to move
armies 1/2 way around the world.
http://en.wikipedia.org/wiki/1914_in_aviation

It is silly to say that moving billions of people off Earth sometime in the future is not possible, and I say within a time frame of 50 years it could be possible, if you shorten that time frame to 10 years, then it is unrealistic. We will be lucky if any humans are standing on
the Moon within a decade- the plan is by 2018. So, by 2017 it like expecting a govt
project to be done under budget. But even if humans return by 2020 to the moon, even
at that snail's pace it's still possible that before 2057, millions of people could traveling to
space and that at that time moving billions of people within a decade of time is something
that could be done. Now, when I say it's possible it is course possible that instead of by 2057, it
could be instead be by 2113.
The critical part is getting to point in which there are say tens of thousands of people
traveling to space per year. And if we have hundreds of people going to space per year,
then getting to point of tens thousand per year, is largely dependent upon whether laws are
passed to stop it, because it's a given otherwise, and could even be given regardless of laws
passed. But you could pass laws that make this happen sooner.
To get to hundreds of people per year going to space, requires that there be a market
in space, and this market has to be free market. You will only get hundreds of people going into
space if it is commercially profitable for people to go into space. In other words, it doesn't
mean much if NASA using tax dollars wastes billions of dollar sending 200 people in space
within a year's time.
But if say, 200 people buy and get a suborbital trip into space that is significant [though it is not very far in space, but skims thru the space environment it's is similar and related]. Hundreds of suborbital flights per year in which some of them are involving traveling more than say 100 miles somewhere, should in within a very short period of time, lead to orbital trips, and orbital trips will then lead to having orbital orbital trips to going to Space hotels, etc.

So, the real significant of this kind of thing happening, is if it's done by private firms
operating in a free market. It means there is practically unlimited amount of money available if something is seen to be profitable and it means these firms will be *driven* by making it safer, cheaper, and more interesting for their customers. Normal way to win is attract more
customers by giving the customer something he/she wants. Their customers will first be people who
are rich, but to reach more customers they will need to lower the price.

Looking at current airline business you have two major factors, price of fuel and getting plane full of passengers every flight. And getting a plane full of passengers is largely about the cost of fuel.

"Compared with aircraft constructed in the 1950s, the improvement in aircraft fuel-efficiency is even more dramatic. In the past 50 years, for example, airplane fuel use per passenger-mile has been reduced by 70%"
http://www.energybulletin.net/18057.html

At Wiki has rough numbers:
It says airplanes get 78 mpg per passenger

Oh, btw also interestingly it also compares air freight to truck freight:
Fuel consumption (BTUs per ton mile):
Heavy Trucks: 3,357
Air freight: 9,600
http://en.wikipedia.org/wiki/Fuel_efficiency_in_transportation

Now, in short trips an airplane doesn't do well as far as MPG, but if airplane is traveling about 2000 miles at 75 MPG, it's about 27 gallon of jet fuel per passenger, and a large part of the cost of the ticket. Now, there are other costs, but a plane costing $200 million that flies 1 million miles per year for 20 years with average of 200 passengers is only costing 5 cents a mile or $3.75 per 75 miles. So that's more than price of jet fuel, but probably most commercial jets get more use than that.

So, whenever space travel price get down near the costs of it's fuel, one can expect a large amount of effort to made to reduce the per passenger cost in fuel.