It will depend upon how much damage was done and the amount it will cost the city in which the accident happened to fix it. If the hydrant itself was just damaged and is the only thing that will be repaired it may cost anywhere from $1000 to $3000 from repair costs we have seen.
Some cities though will bill the at-fault driver that hits a fire hydrant for the labor, equipment and calculated loss of water. So even if it is only a couple of thousand for the actual repair of the hydrant, if there was loss of water than it may cost thousands more.
One city we saw that gave information on this subject said that a good quality fire hydrant is around $1,000 but with the cost of a complete installation comes to about $3,500 but can cost up to $6,500 if other pipes were damaged and streets, sidewalks, etc. have to be opened up and then replaced. If you are charged for any water consumption on top of that it could be several thousands more dollars on top of the repair costs to the actual fire hydrant.
If you have hit a fire hydrant and expect the city to bill you than you can wait for the notice or see if you can contact the right city department to see if they can advise you on the average amount a driver is billed in this type of situation so you can at least have an idea of what you may be charged.
Since the cost to repair a fire hydrant can become very costly you should inform your insurance company since it should be covered under your Property Damage Liability coverage if you hit the hydrant in an auto accident. As for damage to your own vehicle, you would place the claim for damage it sustained when hitting the fire hydrant with your Collision coverage and pay the deductible amount related to this coverage.