I couldn’t tell you
From Longman Dictionary of Contemporary EnglishI couldn’t tell youI couldn’t tell youNOT KNOWused to tell someone that you do not know the answer to their question ‘How much would a rail ticket cost?’ ‘I couldn’t tell you; I always drive.’ → tellExamples from the CorpusI couldn’t tell you• "Is it going to rain tomorrow?" "I couldn't tell you."