It is always advisable to ring Arisaig Marine in advance to book your tickets, particularly if you are a large group.
Arisaig to Eigg return fare £15. Parties of 10 or more will get a group discount. Be sure to ask.
The Shearwater may be chartered for unscheduled runs for about £180. For a group of 10 or more this is always worth considering, particularly for an evening run.