Most of the transatlantic freighters, which carry passengers, either start from continental Europe (so you'd have to travel to a foreign port to board) or go via continental Europe (thus lengthening your journey and increasing the cost).
For example, one route starts from Tilbury (Day 1)and goes to New York via Rotterdam (Day 2) , Dunkirk (Day 4) and Le Havre (Day 5). It arrives in New York on Day 13 of the voyage. Fares are roughly in the range �70 to �90 per day, based upon double occupancy, so a single journey will cost (very roughly) around �1000. Adding on the cost of a flight back (or another freighter trip), the total price might well exceed that of some of the cheaper cruises:
http://www.freightercruises.com/07/05/-news.ht ml
Chris