Dompu is a bit of a transport hub, with through-buses to most mainstream destinations throughout the day. Fares and times below are what we were quoted at the terminal bus station which sits a couple of kilometres to the west of town. You'll need to get an ojek there.
Bima: Regularly from early morning to mid afternoon, taking 1.5 hours and costing 25,000 rupiah.
Calabai and Kadindi (for Pancasila and Tambora): Roughly on the hour from 07:00 to 14:00, taking four hours and costing 40,000 rupiah.
Hu'u (for Lakey): On the hour throughout the day, taking one hour and costing 20,000 rupiah.
Sumbawa Besar: Roughly on the hour from 07:00 to 16:00, taking five hours and costing 60,000 rupiah.