That's quite a distance ohhh man and who can give you an exact price other then somebody living in that area.
Just a guess I'd say 6000. I live on Luzon and a 2 hour trip to Manila and back plus a restaurant and mall stop is 5000 pesos, but my distance is much less and you'll have to cover the ferry cost also.
Does Dumaguete have air-conditioned buses and then are you able physically to travel public transportation and lastly don't they have an airport near Dumaguete, I thought they did.