This site will answer your queries, I hope.

http://www.olymposyachting.com/en/ferry_boat/kos_bodrum.asp

We went 2 years ago,cost about £12 each,there was a special offer on,sometimes its the high speed one,ours that day was the ferry,very nice inside seating,we sat on top going out,rows of plastic chairs.Took just over the hour.Kos is lovely,friendly in the bars and restaurants,and not expensive like Rhodes.We departed from Bodrum at 9am,and I think the departure from Kos was 3.30,or 4pm.Dutyfrees are cheaper over there.

Thanks,
Sounds worth doing.
I love Greece and had considered Kos this summer but have opted for Turkey to sample some different culture....I'm sure I won't be disappointed.
Wouldn't mind a glass of retsina! (Is there a Turkish equivalent?)