The air conditioning system is expensive to repair, as it is a sealed system and needs to have the freon evacuated by a certified mechanic to replace most of the parts. I was quoted $1,500 for a compressor replacement. If the compressor fails, the condenser (in front of the radiator), dryer, and all line seals need to be replaced also.
Some earlier Ford Fusions have an issue with the temperature sensor in the heater box behind the dash fail and will act as yours is. For this sensor to be replaced, it should cost around $500.00 as the dash needs to be removed and takes around five hours to repair. Call the certified mechanics in your area to see if you can get a free proper diagnosis and repair price quote. Good Luck