(that’s a) good question!
From Longman Dictionary of Contemporary English(that’s a) good question!(that’s a) good question!spokenNOT KNOW used to admit that you do not know the answer to a question ‘How can we afford this?’ ‘Good question!’ → questionExamples from the Corpus(that’s a) good question!• "Does the program allow you to do that?" "That's a good question - I don't know."