I have a radiator leak. What should I do?

If you are sure it is the radiator either remove it and have it repaired or replace it with a new or used one. I would not, under any circumstances, use a stop leak product in the cooling system. A stop leak product may fix the radiator, but may plug up the heater core or cause other issues.

Thank you. What is a fair price regarding radiator?

I have no idea on pricing on Audi parts. Perhaps do some "Googling" to see what is available.

If the leak is really small Alumaseal might fix it and I have never had any problems in any car I have used it in. Other products I would avoid though.

I agree with Regrets alumni seal does work if it is a small leak but will clog system if you add more then what is recommended. In other words if it does not seal the leak the first time do not continue to add more hoping it will work.

most rad shops can repair the rad for little cost but for a bit more you can get a new one. If your rad is to the point of leak you might as well just replace the whole thing and call it a day. Cause if its starting to leak now whos to say it won't leak again even after its sealed. Audis are High heat vehicles so fixes don't ussaly go a long way cause parts will just keep breaking down unless its replaced.