Assuming you are referring to a car, it can be difficult to estimate the cost of repairing reverse gear without knowing the make and model of the vehicle as well as the severity of the damage. However, some rough estimates for common repairs range from $200-$2000.
If the gears are simply worn down and need to be replaced, this may only cost a few hundred dollars. However, if the damage is more severe and requires extensive work such as rebuilding the transmission, this could cost upwards of $2000 or more.
Ultimately, the best way to get an accurate estimate for repairing reverse gear would be to take your car to a qualified mechanic or dealership and have them diagnose the problem.