ferry from Italy to Greece

I expect the cheapest will be from Brindisi as it was ages ago when I took an overnight ferry and slept on deck (cold!). Here's the link to check out the routes and fares http://www.ferries.gr/

There is probably not much difference in cost whether you leave from Bari or Brindisi, maybe there is a cost difference if you were to arrive at Igoumensitsa as opposed to Patras, since most boats stop there first, but unless you are doing Northern Greece, it really does not make sense, you will lose it in travel on the Greek side in $$ and days. With the Eurail pass, I believe you get Deck passage with the pass, you still need to go to the office and get a ticket, but there will be no or minimal cost, you may have to use two travel days though, but you probably would use them anyway to get to Bari, then Patras to Athens. Deck passage means that you can be out on the deck, or if you can find an out of the way spot inside, you can crash there. You can also pay an upgrade to get an airplane type seat, a dorm bed, or even a cabin.

I had a deck pass one time on a night ferry going from Piraeus to Crete, and swore I would never do it again. Depending on how full the ferry is, you could get stuck (as I did) with a crowded deck full of smokers. I was miserable the entire time. However, I lucked out when I took the ferry from Patras up to Brindisi - the boat was pretty empty, and although I booked a 4-person cabin (I was traveling solo), I had the entire cabin to myself. Whichever you choose, I would also suggest trying to book your ticket on an Italian line, rather than Greek. Unless things have changed a lot, the Italian boats have more conveniences and are more comfortable.