I took the ferry from Istanbul to Odessa in the summer of 2005. I found the ferry company (Ukrainian) also through ferryline.com The actual trip took 2 days. As I recall the price back then was USD 150 for a single berth in a 4 berth cabin. Don't expect much in terms of comfort, food was acceptable, facilities, well let's just say that you couldn't flush any of the toiletts after the first day. Not much in terms of activities, most just smoke and play cards in the lobby and watch russian tv. But if the weather is nice it is great spending time out on the desk (lots of dolphins following the ship). Apparently there are lots of eastern european working girls in Turkey, as there are many girls taking the ferry back to Odessa. I think the male to female ratio during my trip must be somewhere around 1:5 that really shows comes disco time on board the boat's small bar in the evenings.
This was back in 2005, so things might have improved or changed since then.