I don't know if it's cheaper, but you could look into taking a freighter ship. (I want to take a boat there so bad, so I've been looking at this stuff for like a year now. You can't take kids under 14 on a freighter, which ruled that out for me pretty early.)
You still get meals and stuff, but without all the frills of a regular cruise, and you have to BYOB, but it's still nice.