$15 at most.
Really, $0. It's not worth the time, heartache, or disk space. Check back at first expansion. Assuming Stardock is still in business at that point, of course.
King's Bounty: Crossworlds comes out in two weeks, and Civ5 a week or two after that. Spend the $50 on those (KB:C is $20, and there will be a $20 discount on Civ5 at release in some stores).
Since you're not actually buying a product at this point, and are instead buying the promise of a maybe complete, maybe enjoyable product at some enigmatic time down the line that the echochamber of fanbois swear up and down is coming, you have two essential questions to ask yourself:
1.) Do you want to bankroll Stardock's development? Elemental is not finished. It's not even halfway finished. It's broken on a technical level, but worse, it's broken on a fundamental design level.
2.) Do you want to support these business practices? Stardock deserves to be punished, not praised. Every dollar you give them is the same as saying "I am OK with what you did with this product." If you're the type that thinks what they pulled is OK, by all means, throw money at them. Hell, buy a few copies for friends and family. Maybe you'll push their post-release sales into the triple digits if you do so.