How much bandwidth does a Minecraft client use in SMP?

Well, according to this discussion:

Over a 100second test window, I recorded a mean of 31,630 bytes/sec and a max of 159,371 bytes/sec. That works out to an average of 0.1Gb/hr/user

As a rough ballpark measure that seems fairly believable (this was posted just under a year ago however, so SPM network performance could have changed since then).

Of course it depends a lot on what the users are doing, for example if a player moves around a lot then I imagine that the network use may be greater as the server needs to send more environment data to the client, similarly whenever users connect and disconnect network use will probably spike.

My advice would be to have a go yourself using 100 Mb / hr / user as an initial guess. You should also take into account the distinction between upload and download bandwidth, as many ISPs have a much smaller limit on upload bandwidth.