retrieving material from asteroids

Most studies of O'Neill type colonies assume that we would get the
millions
of tons of material from the moon via a catapult. However, there are some
Near Earth Asteroids where the return dV to the earth-moon space is
astonishingly small. For Nereus, 1982 DB, it is about 60 m/s, or 135 mph.
This opens the possibility of bringing back enormous payloads
on the order of 100,000 tons at a time relatively cheaply.

For example, a small mass driver with a specific impulse of 200 (approx.
2000 m/sec) would need to expend only 3000 tons of material to move it
to the earth-moon space.

However, I don't know enough orbital mechanics to know if we would need
short impulses for part of the return, as I believe it uses lunar gravity
assist. So does anyone know if the low-g acceleration of a small mass
driver would be sufficient, or do we also need higher gee thrust for
part of the mission?

-- Larry Gales