Quick search shows various infographics: green tea at 20–35 mg, black tea at 45–50 mg, espresso at 45–75 mg, and instant at 60-80 mg on average. The day I feel anything close from a cup of black tea to what I get from an instant coffee or a quarter of a pure caffeine pill, I might start trusting those numbers, but for now I see them as nonsense that tries to present as science.