The measures of difficulty of obtaining the water from an object in space include the difficulty of lifting the water off the object, the difficulty of changing the orbit so it passes close to Earth, and the time the payload must spend getting to and from the object.
The amount of material a given rocket thrust can just barely lift off the surface of the object gives a measure of the payoff a mining interest receives when it launches that rocket. The more material a given rocket can lift, the more profit.
The orbital maneuver difficulty indicates a measure of how much of the propellant must be consumed to deliver the payload back to an orbit that crosses Earth's orbit. One measure is the delta_V a rocket and its propellant will impart on the payload/space tanker system. An upper bound on the propellant mass used per payload mass is given approximately by the exponential of the ratio of the total delta_V divided by the propellant specific velocity (Vsp). Steam propellant rockets achieve between 1500 and 2500 m/s Vsp. Hydrogen propellant rockets achieve between 5000 and 9000 m/s Vsp.
[We should consider first] those water sources where some orbital maneuver and rocket combination can bring back the payloads without using more than about 20 units of propellant for one unit of payload. The frequency of orbital alignments and the orbital travel times give measures of economic accessibility. Times less than a few years are essential for reasonable net present value and return on investments. The environment expected gives a measure of the difficulties to be expected in the mining operation.
Water requires only heat energy at less than 800 Celsius for use as a propellant in the thermal rocketsof the space transportation system. It requires only heat energy at about 1200 Celsius when mixed in about equal parts with hydrocarbons to yield hydrogen gas. Hydrogen gas requires only heat energy at less than 2500 Celsius for use as a high performance thermal rocket propellant. Powell et. al. (1993) analyzed the use of water as a propellant in the American Institute of Physics Conference Proceedings # 271, at the Tenth Symposium on Space Nuclear Power and Propulsion, Albuquerque, New Mexico.
The anomalous low vapor pressure (less than 5 mm mercury) of water just above its melting point (0 to 5 Celsius) permits use of relatively low mass container tanks. For example, a thin, mylar bladder can hold 10,000 times its mass as water in the vacuum of space. Such a bladder with mass equal to the 25 tons of the Shuttle payload would hold 250,000 tonnes propellant. This is enough propellant to bring back 10,000 tonnes payload to an orbit around Earth from the "nearby" periodic comets named earlier.
If you see any large holes in the above arguments, we would appreciate hearing about them.