LANE ferry, it goes about 2x per week, takes 17 hours I been told. Actual times/days won't be available until a few weeks beforehand.
Use
www.openseas.gr to see what's available when you need it. You might need to find a ferry that goes to Crete first and then change there to another one that goes to Santorini.
I know from Kos there is one to Mykonos and from Mykonos there's a daily highspeed catamaran to Santorini.
Since prices differ with companies, you should check online who offers the cheapest tickets.
You can do that by Lane Lines ferries
http://www.ferries.gr/lane/
Hope my answer has been helfpul!
Try using the "build your trip" flight comparsion tool here:
Flights To Greece it will
search multiple websites and get you the best price.
Links of interest:
Greece Ferries Rhodes Greece