how much does it cost to get the tire pressure warning system fixed?

It all depends what is wrong. Usually the light will flash if there is a malfunction in the system. If the light is staying on I would recommend taking the vehicle to a tire or repair shop and have all of the tire pressures set. It is very important that they all have the same pressure and that they are not over inflated as well as under inflated.

Was the spare tire pressure checked also????