The thing I can see from these shots is that you have possibly some colour (blue) over the top of dark grey to black potch. This can be good if the colour is clean and has nice play to it. If youve got some wet and dry sandpaper (400 grit) you could gently polish away some of that white and see what the colour is like underneath.
I dont think you can really put a price on opal until you have idea of what youve got so this can give you a start point.
Also you can have a look on the black opals direct website to see what they charge for parcels of rough opal to give you an idea.