We normally set people up with Exchange online plan 1, which comes to around £3 per mailbox per month. The plan 1 only includes mail and nothing else, but there is also a plan 2 which includes office H&B, or plan 3 which includes Office Pro and certain sharepoint and onedrive stuff.
You can also add individual licences for the office packages. With up to 5 installs in any of the plans.
I think you also get 50GB storage with any plan.