Since a reposition cruise is a necessity for a ship, and because they usually occur in the off-season, repositioning cruises are generally cheaper on a per diem basis than the average cruise. Because they offer fewer stops the port fees and taxes are lower. Also, unlike revenue cruises that are routed to the places people really want to see, there isn't much demand for the Azores, the most remote islands in the Atlantic, for example.
I often see Atlantic-crossing repositioning cruises for less than $50/day. This is a real bargain, but you need to choose your ship and itinerary carefully. An older ship with small televisions and not much nightlife might start to feel pretty claustrophobic after the fifth straight day at sea. Plus, if it is sailing in November the seas can get pretty rough.
Few repositioning cruises ever have more than four days at sea in a row, although you might see cruises with four days at sea, a short stop in the Azores, and then four more days at sea. That is a pretty boring cruise.
I have personally crossed the Atlantic three times. The longest stretch I have ever been at sea is ten days, on my very first cruise, from Los Angeles to Tahiti. Yes, I was a little claustrophobic but the whole ship thing was still new to me. The crewmembers that had already been on that ship for several months were far more claustrophobic than I was.