I wouldn't be surprised if you could sell a piece like that for $30 or more with the right buyer looking for rock like that..
Personally that is more than I would pay for it.. but I also wouldn't pay the prices that many people post for "bounce mushrooms, named zoas,etc..."..
Just depends on how fast you want to sell it.. If you can wait you can likely hold out for that buyer that will pay big bucks because its what they want..