The cost to have a transmission fluid leak fixed will depend on the make and model of the car and where the transmission is actually leaking from.
If it's coming from the transmission pan then it will be a more simple fix but if either the front or rear seal is leaking on the cars transmission then they will need to pull the transmission out and separate the engine to get at the front seal and rear transmission seal.
If it's a simple repair it could be from $150.00 to $200.00 to repair the transmission leak but if the transmission leak repair involves pulling the motor and transmission then you could be looking at a $1,000.00 repair bill to fix the transmission leak.