Cost of the parts, which we don't know, plus the shop's hourly labor rate, which we don't know. Any shop will give you a cost estimate. It will vary too whether you simply request to have a specific task performed or if you want the problem diagnosed. If a shop already diagnosed the need for a new catalytic converter they would have given you an estimate at that time. Did they also diagnose the cause of the failure? If not, they only did half of the job. If you don't like the estimate, get another diagnosis from a different shop. If there's a substantial difference, compare what is different between those two estimates.