failed emission because of P0442. A small leak was detected. on 2001 Toyota Camry

I was told to repair or replace the canister/purge valve. If I were told to replace those parts, should I pay for a general diagnostic then have the part repaired? how much does a repair or replace costs? The light came one once before and I replaced the gas cap. It did not go off but was told by the person at Auto Zone that the gas cap was the problem.

Asked by for the 2001 Toyota Camry