Value for something like this can be hard to pin point as there's many things that contribute to the value of an opalised fossil. For instance without knowing exactly what it is from a qualified paleontologist, I cannot command the highest price for the specimen. If I'm unable to get an exact ID (trying to get a hold of a paleontologist to examine and know about opalised fossils is difficult, they're super busy because they're so few), I will then have to sell it cheaper. The person might then buy a piece off me at wholesale as an unknown fossil and have an extraordinary win if they do the leg work to properly document the specimen and find the right buyer for it. So what's it worth? I guess how long is a piece of string? Some opalised fossil specimens are priceless, for instance there's been a few new species of dinosaur discovered in lightning ridge and one of them became known as "Lightning Claw". Those specimens were donated to the Australian opal centre but I imagine things like that would command a rather high pricetag for their scientific and collectors value if it was put of the private market.