Hop David wrote:

If you achieve LEO with an empty fuel tank you're not halfway there.

That depends on the payload.

Once you reach orbit cheaply you have a lot more options.

For example ion drives are not very expensive to launch, because they
use very little fuel.

Of course using ion drives for people is problematic due to the Van
Allen belts and because they can be rather slow, but there's many
tricks; for example you can use the ion drives to ferry around chemical
fuel. That way, the chemical rocket only needs to carry enough fuel to
reach the next fuel dump. Doing that linearises the rocket equation; it
saves lots and lots of fuel on high delta-v missions like going to Mars
or landing on the moon.

Hop
http://clowder.net/hop/index.html