As far as I'm aware, they typically aren't able to tell people about sales before they occur. Going into the holiday season there's a good chance that it will be cheaper at some point, but there's no guarantees.
Plus, I completely second it being well worth $10. I'm not a big puzzle game fan and typically get bored with them quite quickly, but Tidalis held my attention through the entire adventure mode and then some. Easily worth the money.