Hey,
I am not aware of any Liveaboards operating from the Island itself, there are a few centres in the south of the Island who, last I was aware, provide accommodation, air and boats out to sites. I will post some addresses if I can find them. But as regards prices, I think for this trip you will have to be prepared to pay. Travel on/off the Island is not cheap as the best of times -
www.steam-packet.co.uk for ferry information.
All the Best,
Ali