The Indians are getting MAD on their P8s,
the Americans ummed and aahed for ages before the decision not to have MAD fitted. This was due to the fact that the Americans envisage doing higher level ASW which precludes the use of MAD. High level ASW will create more problems than it solves, the main problem it solves for the Americans is, the P8 don't like it down low.
So for 6 hours
2 x Predator B's for sonobuoy drops,
1 x Predator for weapons carriage and
1 x Predator for MAD ( just to keep the Indians happy), RADAR and EO.
Only the sonobuoy carrying predators would need to be relieved for replenishment.
So for 24 hour coverage, I make that 10 predator sorties to track one submarine. (Assuming the RADAR and Weapon carriers have 24 hours endurance.
Dipping UAVs would mean a rotary type UAV, which is being investigated for use in the P8, and would need to be dropped from a fixed wing platform due to range and speed etc.
10 Predators would be cheaper than 3 MPA, the problem would be the bandwidth needed to operate the UAV swarm and transmit all the data back to be analysed.
This number of UAVs could be reduced with bigger payload carrying variants as LJ suggests.