You need to have the AC system diagnosed and you can check prices here http://repairpal.com/estimator?car_type_id=3228 and locate a shop near you here http://repairpal.com/directory?address=23401&car_brand_names=Lexus Tests have to be performed to know exactly what failed to give a accurate estimate
That happened to me and Had to replace the compressor and flush the AC system. My mechanic charged me about 1500 dollars. If you just replace the compressor your AC system will break down again in about a month. That happened to me too, so be ware.