How much does it cost to deal with my tree stood in water after the yard flooded and now it looks sick?

Most of the useful spending here is not tree work — it is drainage. Expect a tree service to assess and then defer the actual fix to grading, downspout extension, a French drain, or a sump redirect, priced by a different trade. Tree-side costs are root collar excavation to check for a buried collar and collar rot, compaction relief, mulch, and later deadwood removal. If the tree does come down, the cost driver specific to this symptom is ground condition: a saturated yard means no truck or lift access without matting or plywood, or a full climb-and-rig job, and a stump grinder working in mud does poor work and leaves ruts. Waiting for the ground to dry usually costs less, and a good company will tell you so.