The maximum distance for asteroid mining 3 million miles (0.03 AU) with current technology. The corresponding delta-V is 437 meters per second. The near earth asteroids that are in range should contain more than one million liters of water.
Swarms of smaller spacecraft to perform mining is more likely to result in an economically feasible operation. 250 spacecraft fleet collecting 100 kilograms of water per trip would require an investment of about $4 billion and would become profitable after about 9 years.