This one shows it's in the range of 110 euros per day + to do a container ship. Unlike cruise ships, that's basically bed + 3 meals a day. No food 24x7, entertainment, etc. But that would be fun, just relaxing without much to do to write, read, etc.
If someone is into relaxation, long distance Amtrak may be another alternative. I've done it a few times and it's fun to relax, chat to people, read, eat in the dining car, etc.