This is a difficult question to answer because there are so many factors that go into AC repair. However, we can give you some ballpark figures that can help you get an idea what type of cost you might be facing based on what needs to be fixed:
Refrigerant leak detection/repair - $225-$1600
Thermostat replacement - $115-$470
AC refrigerant charge - $100-$800
Condensing unit fan motor - $100-$700
Compressor replacement - $1350-2300
If you think something on your air conditioning unit needs to be replaced, have an HVAC expert give you an estimate so you know where you stand.