2004 Toyota Prius A/C problem

I bought this car 2 weeks ago, and it had been running perfectly, when the A/C went warm (3 days old). Does anyone have an idea as to how much $ the dealer would charge to have the evaporator replaced. I did bring it to my local A/C mechanic (he said the whole dash had to be removed), he put more gas and it worked for 2 weeks after that. I'm inclined to think that its the evaporator....any ideas as to how much $$$ or any sugestions are recomended. Oh almost forgot the mechanic did say that this car uses a special oil lubicant for the A/C and if another oil is used the car could burn up.:frown: