Depends entirely on what character you want to bot -- a decent barb runs 10-20m for roughly 500k~ per hour (at paragon 100).
I used to bot DH's (when Sarkoth farming was king), they're still cheap to gear up and good for straight gold farming (mp10 hell, nightmare etc).
WD - Not sure
Monks - 20-50m
Wizard - wouldn't recommend.
and your farming with gold find gear?