> In comparison how much bandwidth would a Tor bridge user per month?
It would be tough to give an exact answer to that is it depends on how many people are connecting through your specific bridge. However in general the bandwidth requirements would be much less, as bridges are used for mostly areas where the tor network is blocked whereas one of the exit relays is used by every client that uses tor at all.
You can find more information on this topic here
This page contains information about how much traffic different types of relays generally send and receive over time, etc.
PS, my apologies if this comes through twice for some reason. My email client is acting a bit strange at the moment.