Evaporator Core

AC stopped working. We had a purge and refrigerant fill but failed after a week. Firestone said they had done a dye test and found no leaks and now this! Now they are saying I need to have my evaporator core replaced at the dealer. I’m taking to my Nissan Specialist who did our head gasket. Man this Cube is costing us a time of money. Anyone had their evaporator core replaced? How much was it? I think the part alone is over $600 and the dash needs to come off.