the cost would be much less than you think. there are only bullets, grenades and smoke bombs in the turret so no really nasty stuff.
to recover it i would dive down myself (only 20m), connect hydraulic jacks between the hull and turret then go back up to the boat and fire up the hydraulic power pack which would be connected to the jacks. pop the turret off go back down connect the lifting bag, approx 4cubic metres should do it float it off, tow it behind the boat to the slipway at high tide, moor it until the tide has gone out then pick it up with my bedford and be back home in time for tea and medals simples