P.S. Just took the fuel for the tugs themselves into account. Result: Not worth it. Or rather, you need higher water proportions. And you need to go for Lagrange points. And space stations.
Workable scenario A:
- A 7,000 ton in-systems tug works getting asteroids to the L1 point of the local gas giant.
- They basically grab the asteroid and give it a nudge, not thrusting until needing to brake into position.
- These maneuvers burn only 6 burn-days of fuel (36.5t) to move the asteroid over a distance of 7.25 AU over more than 3.5 months.
- A space station at the Lagrange point folds the asteroid into a gantry assembly for disassembly.
- The asteroid - using C1 carbonaceous asteroids - is then disassembled over the next 4 months.
- About 425,000 tons of waste material - olivine - have to be moved away from the Lagrange point every year.
- Result yield for refueling operations is about 12,000t fuel and 1,500t of usable metal for export.
Workable scenario B:
- Two 7,000 ton in-systems tugs tag-team each other getting asteroids to the zenith or nadir jump point of the star.
- They basically grab the asteroid and give it a nudge, not thrusting until needing to brake into position.
- These maneuvers burn 650t fuel per year to move the asteroid over a distance of 10.4 AU within two months.
- The tugs burn another 6,550t fuel per year holding the asteroids in position at the jump point
- The asteroid - using only C1 carbonaceous asteroids - is then disassembled over the next 2 months.
- About one million tons of waste material - olivine - have to be moved away from the Lagrange point every year.
- Result yield for refueling operations is approximately 17,000t fuel per year, additional exports are minimal at 1,000t metal.
Assuming a cost in the region of 140 million for such a in-system tug - primarily in its engine - and an at least comparable price for the space station for scenario A, the above has a benefit-cost-factor of 0.75 for scenario A and 1.04 for scenario B versus Poseidon. Or in other words - a Poseidon can be cheaper, assuming the cost for the ground installations of a Poseidon also needs to be invested in some way for the two scenarios at equal proportion.
Scenario A can be expanded with more tugs feeding the station assuming the inherent capability exists in it; at two tugs we get a benefit-cost-factor of almost 1.0 versus Poseidon, at three tugs we're at 1.1. We'll quickly run into overcapacity issues here though, since with three tugs we already maintain as much capacity as a 8-unit major Poseidon installation. Scenario B has the overcapacity issue from the start, at near-equal costs to Poseidon; it is a viable alternative proposal for 4+ Poseidon sites.
Edited: Revised/recalculated numbers.