I wonder if the downstairs neighbours were not complaining of an actual drip of water into their unit but the sound of a drip. Perhaps the drip into the bath could be heard in the unit below, which would explain why replacing a washer was enough to fix the neighbour’s complaint. The high cost for a washer replacement might be explained by the time taken to work it all out. It sounds to me like the cost was incurred because of impatience when the upstairs people said they were going to get their dripping tap fixed.