Q: Add refrigerant to a/c unit, how much$$.

The best thing to do in your situation would be to have an a/c performance test done.

This test will use a refrigerant recycling machine to test the high and low pressure sides of your a/c system to lead the mechanic in the right direction as to what the issue may be.

This machine can also draw the system into a deep vacuum to remove any moisture that may exist. After the system has been drawn into a vacuum, a vacuum hold test will be performed to test for leaks also.

If the system fails the vacuum test, the mechanic may then add a little bit of refrigerant to the system with some florescent dye. Once this die has circulated through the system, the mechanic can use a UV light and some UV glasses to search for traces of dye that would indicate a leak. After the leak has been located, the leaking part should be replaced. After this has been performed, the a/c machine will be able to fill the system back up with refrigerant.

It would be difficult to tell exactly how much this repair would cost, as I am unable to tell the exact source of the problem. However, if your a/c system is low on refrigerant, it is most likely due to a leak. A/C systems do not just lose refrigerant over time any more.