If you’re going to Sakurajima, I think it’s best to get the one-day pass! It’s only 500 yen and you can use it for the entire day, just hope on and off the bus. When getting off just show your pass to the driver. When you first arrive into Sakurajima at the ferry port, you can buy the pass from the tourist office, they will also provide a print out of the timetable. If you want to use the buses without the pass, it will cost between 120 – 440 yen depending on the distance you have travelled.
As far as I am aware, there are 2 bus routes. A and B. I remember one of them will turn in to go straight to Yunohira and the other will take you down to the Research Centre. I think that was the only difference but all routes are looped and will return back to the ferry port. It’s a fun way to see the important parts of the island. I’d like to also point out that once you reach Yunohira, the bus driver may take a short break here and may leave both doors wide open….The ashes from the volcano might get blown into the bus by the wind.