The crossing at its shortest is about 14km (close to 9 miles I guess). We did the crossing in fog from Bottle Point (we had taken some solid compass bearings the night before) without any problems, but the way back (across to Jackfish Bay) was spectacular and calm. The Rossport area does have enough spectacular paddling to take up a week with the family, but the Slates are somewhere special to be sure. The caribou are a really unique drawing card, but there are some beautiful beaches, diverse geology, old mines, a lighthouse, and some historic camps, buildings, and shipwrecks (including a brothel).
Naturally Superior Adventures in Wawa may be able to set you up with a group already shuttling across to save cash, you may be able to find groups through various message boards, or you may just want to ask the operator if he has a group going you can combine with to save everyone some money.