Let me address a couple of matters of terminology first.
(Unless you need a manned mission or have other factors which impose timing constraints) distance doesn't matter. It is $\Delta V$ (change in velocity) times the mass that needs to move that constrains the mission.
To find the most economical method to move things around, consider the total amount of $\Delta V$ required to accomplish each goal.
How much $m_{equipment} \times \Delta V$ must go out?
How much $m_{asteroid} \times \Delta V$ must come back?
Add the two sums together for the total mission budget.
To minimize program costs, you want to minimize the total mission budget.
Mine in situ or bring it back?
If you need lots of equipment to accomplish your mission objectives (e.g. you require a manned habitat and must keep it supplied) you may want to bring the asteroid back to minimize this cost.
Alternatively, if you want the entire mass of the asteroid brought back (e.g. to form the core of a manned habitat), then you will need to return the entire asteroid's mass.
Otherwise, you will probably want to mine it in place.
If time doesn't matter, you can reduce $\Delta V$
If you're just shipping raw materials around, then you can save significant amounts by using the Interplanetary Transportation Network
(the pinches in the tube above are representations of the Lagrange Points
The Interplanetary Transport Network (ITN)1 is a collection of
gravitationally determined pathways through the Solar System that
require very little energy for an object to follow. The ITN makes
particular use of Lagrange points as locations where trajectories
through space are redirected using little or no energy. These points
have the peculiar property of allowing objects to orbit around them,
despite lacking an object to orbit. While they use little energy [aka
$\Delta V$], the transport can take a very long time.
Basically, you pay the full cost for a Hohmann Orbit Transfer to one of the ITN keyholes and only a very small additional cost to steer the object through each ITN keyhole as your asteroid passes through it.
Where do you get your $\Delta V$?
Current human spacecraft bring their propellant with them. To supply the propellant necessary to develop the $\Delta V$ for your mission would make the mission impossible. So we need to find it somewhere else.
The main contenders that I see are:
- Use materials from the asteroid as propellant (conventional chemical
/ nuclear rocketry)
- Mass driver / coil gun the materials back (e.g. use electromagnetic
devices to launch the materials onto the correct course - uses the
asteroid as its propellant)
- Use photons, solar wind, magnetic fields as propellant (various sail
concepts)
- Use a composite solution (solar focusing on asteroid to use asteroid
mass for propellant)
Getting positive Return on Investment
Anyone deciding to engage in such an endeavor wants to earn more back than it costs to do. The longer the mission takes, the higher a return the investor will want to see.
As I see it, other than as novelties (very small volume of sales) and scientific research, asteroid raw materials cannot compete with terrestrial supplies of those materials in the Earth market place.
Asteroidal materials only have an advantage over terrestrial materials when they are used in space. So we need to develop a space infrastructure that will consume these materials. IMO the first two such infrastructure projects would have to be satellite repair and replacement (probably starting with simply refueling operations) and Earth orbit clean up (removing debris from Earth orbit from previous generations).
So initially we will want asteroidal materials for fuel and building relatively simple "orbital space tugs" designed to push objects around and/or refuel them. Once operations get going and money starts coming in, then we can look for other money making opportunities.
