Don't recall if there are any ferries interisland.., but I recall camping in the state campsites is cheap. They used to be free up to a few years ago but I don't recall how much they are now. As far as stealth camping options, there are people out living in the backcountry in Kauai. When my sister and I hiking the Kalalau trail all the way out to the beach, we ran into a few "locals" living off the grid and in the backwoods...

The only thing I've heard negative about the state campgrounds is that since it is an island, it really isn't too far secluded from "civilization" which includes drunk teens, etc. etc..

Jay