I've never seen that much discount for RT. If anything, it's usually been maybe $20 less. I've never found it worth the hassle and risk. You could lose the ticket. Or you could arrive 10 minutes before the *other* ferry departs but an hour and 10 minutes before your ferry departs. That would be frustrating.
Or what happened to me the entire ferry was reserved for cruise ship passengers, so I had to wait 90 minutes for the next ferry.