What is the estimated cost to replace leaking Air Conditioner return line.

I cannot give you an exact estimate but I can tell you what is needed to complete this job. Always remember that once you open the A/C system you must first put the system under vacuum. This is how you get the proper amount of freon into the system. Of course refrigerant is a better word as freon was a brand name. A dye can is always a good idea to add when recharging the system. If you have any future leaks It wil be quite evident when using a black light the dye will light up and indicate where the leak is located. Then the refrigerant is introduced under vacuum, about 2-5 pounds depending on the model car. I estimate the hose replacement plus labor and cost of refrigerant is about, $200-$300. If you know which line is leaking call around and ask what estimates are at different repair shops. It could vary by as much as 20-30%. Make sure you can get a warranty on the work performed IN WRITING.