Hamburg Süd has freighter cruises leaving from a number of European ports with itinerary lengths of 28-49 days. Rates are expressed in daily costs.
Maris FreighterCruises.com offers trans-Atlantic itineraries that explore a number of American ports, including Fort Lauderdale, New Orleans and Savannah. Daily charges for these trips can run as little as €90 ($120 USD) per day. That's about the price of a discounted traditional cruise. Beware: these trips also require "port and vessel fees" that can exceed €200 ($268 USD) on some trips.
Note that on these trips, the itineraries often begin and end in Europe, with multiple stops in North America. For those who want to start and end a freighter cruise in North America, a lengthy (think months) commitment of time usually is needed to sail to another continent and back again.
In the next frame, take a look at some options for South America.