if they are saying that the ECU is faulty then �500 may well be the going rate for a new one from the dealer, although im quite sure you would find one cheaper second hand on ebay or a breakers yard
very common problem with these, the engine ecu breaks down leaving the engine running on only 2 cylinders,�500 is about right for a replacment for this.
the ecu is coded to the car so you would not be able to use a second hand unit