http://en.wikipedia.org/wiki/Escape_velocity says:
    On the surface of the Earth the escape velocity is about 11.2
    kilometres per second.

You have: 100 kg * (11.2 km/sec) * (11.2 km/sec) / 2
You want: kilowatt hours
        * 1742.2222
        / 0.00057397959

So 1700 kWh per (large) person, to lift them out of Earth's gravity
well (assuming perfect efficiency, as with a space elevator.)

http://www.ecoworld.org/energy/EcoWorld_Energy_Resid_KWH_Prices.cfm
lists average US residential electricity prices from 6.5 to 14.8 cents
per kWh, with an outlier at 33.3 in San Francisco during the
California energy crisis.  It also claims that the cost of the fuel
alone amounts to about 0.5 to 1 cent per kWh.

So if we have to pay 10 cents per kWh, lifting a person into space
should cost around $170 --- an energy cost that could in theory be
recovered if they came back down.  (At present this energy is mostly
dissipated thermally.)

Evacuating the entire human race to an extraterrestrial habitat
prepared to handle them should then have an energy cost around $1
trillion.  This is roughly 2% of annual world GDP ($55.9 trillion) at
PPP.  (See http://www.worldbank.org/data/databytopic/GDP_PPP.pdf for
details.)

Current world energy usage is around 354 exajoules
(http://energy.er.usgs.gov/products/Papers/WMC/17/) or 400 exajoules
(http://www.pugwash.org/reports/pac/agra/agra_reports_wg4.htm) or 375
exajoules (http://www.globalfuture.com/0002.htm) or thereabouts.  1700
kWh per person is

You have: 1742 kilowatt hours * 6 billion
You want: joules
        * 3.76272e+19
        / 2.6576519e-20

38 exajoules, a significant fraction of world yearly energy usage, but
far from unimaginable.  It would probably be enough to raise the unit
price of energy.

I conclude that, while technical obstacles currently make evacuation
of Earth's population to extraterrestrial colonies impossible, the
energy cost of the evacuation itself is within reason.