This bugged me too, and then eventually I realized that the town being upgraded is selected, so you see the resources display on the bottom in the UI's overlay. It's super easy to overlook, though - I still sometimes forget for a bit when the question comes up:-( It would be more effective placed in an open book display similar to when we view the city details (replacing 'Train', 'Build', 'Raze' for this usage).
[copied my answer from another thread with the same issue]