There is no fixed answer to how much data will be transferred during a send/receive - it depends on how many changes the user needs to receive and how many it needs to send.
The changes only contain the differences and these are in a compressed format, so I would think 1MB would represent some large changes. If the users do send/receive every day, the data will be related to the number of changes made that day and I would expect it to be small in most cases.
There is some overhead in getting information about the user’s projects from both the registry and the send/receive server, but that should be small for the average user who is only a member of a couple projects.
Maybe others can weigh in who have actual experience with a metered connection.
John+Wickberg
Paratext Support