A full sleeve is basically a combination of tattoos that take up the majority of the space on your arm. They cost the most because they take the most time to create. The creation of a sleeve tattoo can take anywhere from 5 hours to 25 hours to complete, all depending on what type you get. Most likely it will be completed over a few sessions.
You will usually be charged by the hour by your tattoo artist, the price range is pretty wide from dirt cheap to very expensive. If you get an expensive tattoo artist expect to pay hourly. Sometimes as much as you would pay a plastic surgeon. Most of the time the higher priced tattoo artists produce the best quality work and its why they are priced high.