I read an article about this somewhere yesterday (can't remember where). It said that the actual annual cost of operating the airfield was at or below $1 million; the higher Interior Department numbers apparently included the costs of running their services there in addition (IIRC, a wildlife sanctuary, etc.).

At <= $1 million/year and at least several airlines affected, I would be surprised if the affected airlines didn't join together to pay the cost of keeping Midway alive.