Eliza - you should understand that most ships only cross over (once) and back (once again) every year, so there are not that many cruise options for transatlantic - and almost all of them go in April, (over) but come back between September and November.
Cunard is the only line that "crosses" several times each year.
I can't imagine fees & taxes making the price twice as much, maybe you did not realize double occupancy would be added in when you price a cruise, even as a solo. It is mandatory.
That roommate deal above looks like a very bet for you - a LOT of people do it. Check out our "seeking cruise companion