For this to work it couldn't be soldiery tomes. It would have to be a currency you got specifically for clearing Coil. Moreover, the cost would have to be at least what you'd get for clearing all 4 turns, but more probably what you'd get for clearing all 4 turns 3-4 times. This would essentially provide 8 extra drops per group every 3-4 weeks (and optimum drops at that). It might be the sort of thing that can add later in a raid's life cycle (right about now for SCoB I guess), but near the beginning it would just get people geared up much more efficiently.