I never have a problem finding rather nice traditional cruises to nice locations with full service (food, waitstaff, maid service, entertainment/shows, etc) for well under $120 per night. I've looked into freighter cruises, but it seems like there isn't much of a cost savings with them vs. a traditional cruise.
Maybe if you like solitude the freighter is the way to go, although there's usually a nice, quiet place on every cruise ship (you just have to find it). I remember it was deck 11, aft on the Celebrity Century we took for our honeymoon cruise. Quiet, open air cafe that was usually deserted. You could look out and watch the sun set, and watch the ship's wake work it's way to nothing on the horizon.