A/C does not work ...

system could simply just need to be recharged.. check that it holds a charge and have the tech to fill it with dye so that if there is a leak it will be easy to locate. or it can be any number of problems like the compressor is not working that is very costly . hopefully it is as simple as a charge..

You can check the AC repair costs here https://repairpal.com/estimator/mercury/sable But if its the evaporator its a huge job!