"Delta-v is computed following the approach described by Shoemaker and Helin (1978),
Earth-approaching asteroids as targets for exploration, NASA CP-2053, pp. 245-256."
&
"For comparison, delta-v for transferring from low-Earth orbit to rendezvous
with the Moon and Mars:
Moon: 6.0 km/s
Mars: 6.3 km/s "

"1 99.98 1991 VG 3.998 0.666 0.635 28.5 1.027 0.049 1.4
2 99.96 2006 UB17 4.082 0.680 0.648 26.3 1.140 0.103 2.0"

And for rest of the 4,200 of them, see:
http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html

So from LEO the rock with least delta-v is 3.998 Km/sec, though I believe it's possible that 1991 VG is spent Apollo stage, and next closest is 2006 UB17 with 4.082 km/sec.

Now, if you want more exact or more detail about how this delta-v calculated you probably need to look at that "Shoemaker and Helin (1978)" reference.

Roughly, speaking I would say from high earth orbit, L-points or Lunar orbit there appears to be about hundred NEOs which could be reach with less than 2 km/sec. Or if starting from LEO about 642 NEOs which require less delta-v than 6 km/sec [less than the 6 km/sec needed to land on the Moon].

Now the Mars 6.3 km/s is a trajectory which takes something like 9 months and one has with Mars trajectory a launch window every two years or so. A NEO might be similar- have windows every year or so and the travel time could be months to get to it. And like Mars one could choose trajectories at different launch windows which could result in shorter travel time [and higher delta-v] or longer travel time with around the same delta-v [the advantage being you have more launch windows and/or get to Mars in different orbital locations [say you want arrive at say the southern poles' summer].