2000 Cadillac DeVille DTS 57000 miles blowing hot air

My 2000 Cadillac Deville DTS has 57000 miles is blowing hot air. The code DTC P1540 ACX refrigerant Overpressure -Air Conditioning A/C disabled. I called the dealer and told them that my car is blowing hot air. They told me that in order to diagnose my car, it would cost $180.00. Immediately they would evauate the freon. I purchased an A/C pressure valve. Can that valve be installed without evacuating the freon? How do I get them to take a look at my car without charging me anything? Why should evaucuating and replacing freon cost $180.00 or more. Does anyone have an excellent cadillac mechanic that has his own shop in new jersery? I am a woman and I feel that I am being taken advantage of. Thanks